Surgical excision versus clinical follow-up: Outcomes of benign intraductal papillomas diagnosed on core needle biopsy

OBJECTIVE

To compare outcomes between benign intraductal papillomas diagnosed on core need biopsy that were excised (BIP-E) versus those that were followed-up (BIP-F) at our institution.

METHODS

Patients were identified by an electronic data base search from January 2010 to October 2016. After exclusions, clinical, radiological and histologic variables were evaluated and biopsy and excision slides reviewed.

RESULTS

110 BIP from 104 females were analyzed. 84 BIP were excised and 26 BIP were followed up (mean 43.3 months, range 7-93 months).11 patients in BIP-E group had atypia on excision. There were no statistically significant differences between BIP-E with atypia and BIP-E without, except for clinical presentation with pain/discomfort (p ​= ​0.015) in the former. There were no true upgrades to malignancy in both groups on follow up. One patient from each group developed a new breast cancer distant from IP site after nearly 4 years of uneventful follow-up.

CONCLUSION

Clinical follow up is an oncologically safe alternative for radiologically concordant BIP. Excision may be considered if a diagnosis of atypia would impact surveillence and chemoprevention recommendations.

Synovial Biopsy in the Diagnosis of Crystal-Associated Arthropathies

BACKGROUND/ OBJECTIVE

This study seeks to assess the utility of synovial biopsy in the diagnosis of crystal-associated arthropathies (CAAs) in a clinical setting.

METHODS

In this retrospective study, we reviewed biopsy reports involving synovial tissue between 1988 and 2015. We then reviewed the records of patients where the biopsy was performed for a clinical suspicion of CAA-the clinical group-and calculated the frequency of a positive diagnosis. The t test, Mann-Whitney-Wilcoxon test, and Fisher test were used to compare clinical characteristics of patients with and without a tissue diagnosis of CAA. We also reviewed cases of unexpected detection of crystalline disease involving synovial tissue-the incidental group.

RESULTS

Among 2786 biopsies involving the synovium, we identified 65 cases in the clinical group and 33 cases in the incidental group. In the clinical group, a relevant diagnosis was obtained from synovial tissue in 36.9%, and a CAA was diagnosed in 20%. Restricting analysis to clinical biopsies performed for a primary suspicion of CAA, a relevant diagnosis was obtained in 61.3%, and a CAA was diagnosed in 38.7%. The incidental group comprised 1.2% of all synovial biopsies and included 7 mass lesions. Basic calcium phosphate was not reported on any biopsy in the study period.

CONCLUSIONS

Synovial biopsy is a diagnostic option when suspected CAA is resistant to conventional modes of diagnosis. Crystalline diseases should be considered in the differential diagnosis of musculoskeletal mass lesions mimicking neoplasms.

Trans-caval trans-jugular liver biopsy--a technical modification of trans-jugular liver biopsy

OBJECTIVE

To (a) describe the technical modification of trans-caval TJLB and (b) review our series of nine cases.

METHODS

We performed a retrospective review of all trans-caval TJLBs performed; we assessed indications for the procedure, technical success, complications, adequacy of specimen and histological positivity.

RESULTS

The technical success rate of the procedure was 9/9 (100%); the minor complication rate was 1/9 (11%), adequate specimen was obtained in all cases and a histological diagnosis was achieved in 8/9 (89%) cases.

CONCLUSION

This preliminary report suggests that trans-caval modification of TJLB is a relatively safe procedure that may be useful in cases where conventional TJLB is infeasible.

ADVANCES IN KNOWLEDGE

(a) We describe the technique of trans-caval TJLBs and report our findings in the largest series of published cases. (b) Trans-caval TJLB is relatively safe and can be used to increase the success rates of conventional TJLB.

Errors in potassium measurement: a laboratory perspective for the clinician

Errors in potassium measurement can cause pseudohyperkalemia, where serum potassium is falsely elevated. Usually, these are recognized either by the laboratory or the clinician. However, the same factors that cause pseudohyperkalemia can mask hypokalemia by pushing measured values into the reference interval. These cases require a high-index of suspicion by the clinician as they cannot be easily identified in the laboratory. This article discusses the causes and mechanisms of spuriously elevated potassium, and current recommendations to minimize those factors. "Reverse" pseudohyperkalemia and the role of correction factors are also discussed. Relevant articles were identified by a literature search performed on PubMed using the terms "pseudohyperkalemia," "reverse pseudohyperkalemia," "factitious hyperkalemia," "spurious hyperkalemia," and "masked hypokalemia."

Enterococcal Bacteremia is Associated with Prolonged Stay in the Medical Intensive Care Unit

Background:

Although enterococci are relatively common nosocomial pathogens in surgical intensive care units (ICUs), their significance in blood cultures from patients in the medical ICU is unclear.

Materials and Methods:

In this retrospective study spanning 2 years, the clinical and microbiological characteristics of enterococcal bacteremia among medical ICU patients were evaluated.

Results:

Of 1325 admissions, 35 with enterococcal bacteremia accounted for 14.8% of positive blood cultures. They were significantly older (P=0.03) and had various co-morbidities. Most had vascular (96.9%) and urinary (85.3%) catheters, and 67.7% were mechanically ventilated. In addition to blood, enterococci were isolated from vascular catheters (8.6%) and other sites (20%), while no focus was identified in 77% of patients. Prior use of broad-spectrum antimicrobials was nearly universal. All isolates tested were sensitive to vancomycin and linezolid. Resistance to ampicillin and gentamicin were 44.7% and 52.6%, respectively. Compared with other medical ICU patients, patients with enterococcal bacteremia had a longer ICU stay (P<0.0001) and a trend toward higher ICU mortality (P=0.08).

Conclusions:

Enterococcal bacteremia is an important nosocomial infection in the medical ICU, with a predilection for older patients with multiple comorbidities. Its occurrence is associated with a significantly longer ICU stay and a trend to a higher mortality. The choice of antibiotics should be dictated by local susceptibility data.

Azathioprine induced cholestatic hepatitis

We report a case of cholestatic hepatitis developed one week after exposure to azathioprine. The subsequent prolonged cholestatic phase was followed by full clinical remission. Current knowledge on pathogenesis and epidemiology and the diagnostic challenges presented by this rare complication are discussed, followed by recommendations for monitoring and management.

Carcinoma of gall bladder presenting as dermatomyositis

Cancer-related muscle diseases are usually paraneoplastic disorders. Dermatomyositis (DM) is a type of inflammatory myopathy that is strongly associated with a broad range of malignant disorders. The malignancy can occur before, concomitantly or after the onset of myositis. The malignancies most commonly associated with DM are carcinomas of ovary, lung, stomach, colorectal and pancreas, as well as non-Hodgkin’s lymphoma. An association of DM with carcinoma of the gall bladder (GB) is extremely rare with only two previously reported cases in the literature. We report a case of carcinoma of GB with DM as the paraneoplastic manifestation.

The Relation Between Chemical Structure and Uncoupling Activity in Congeners of Salicylate

The relation between chemical structure and uncoupling activity in congeners of salicylate has been studied by means of their effects on the oxygen consumption of a suspension of baker's yeast utilising a limited quantity of glucose. Salicylic acid, salicylaldehyde, 2-hydroxyacetophenone, salicylamide, 3-methylsalicylic and 1-hydroxy-2-naphthoic acids were found to show uncoupling activity.

Jejunal access loop cholangiogram and intervention using image guided access

Jejunal access loop is fashioned in patients who undergo Roux en Y hepaticojejunostomy and biliary intervention is anticipated on follow up. Post-operative study of the biliary tree through the access loop is usually done under fluoroscopic guidance. We present a series of 20 access loop cholangiograms performed in our institution between August 2004 and November 2008. We aimed to evaluate the safety and efficacy of the procedure and to highlight the role of CT guidance in procuring access. Access loop was accessed using CT (n = 13), ultrasound (n = 3) or fluoroscopic guidance (n = 4). Fluoroscopy was used for performing cholangiograms and interventions. Twelve studies had balloon plasty of the stricture at anastomotic site or high up in the hepatic ducts. Seven studies showed normal cholangiogram. Plasty was unsuccessful in one study. Technical success in accessing the jejunal access loop was 100%; in cannulation of anastomotic site and balloon plasty it was 95%. One case required two attempts. Procedure-related complications were not seen. All patients who underwent balloon plasty of the stricture were doing well for variable lengths of time. Access loop cholangiogram and interventions are safe and effective. CT guidance in locating/procuring the access loop is a good technique.

Amikacin-induced type 5 Bartter-like syndrome with severe hypocalcemia

Aminoglycoside-induced renal toxicity is well known and may manifest with nonoliguric renal failure or renal tubular dysfunction. Aminoglycoside-induced renal tubular dysfunction could result in diffuse damage or manifest as a Fanconi-like syndrome, Bartter-like syndrome, or distal renal tubular acidosis. We discuss a patient who developed severe renal tubular dysfunction secondary to short-term therapy with Amikacin, resulting in refractory hypokalemia, hypocalcemia, hypomagnesemia, metabolic alkalosis, and polyuria. This constellation of biochemical abnormalities mimic Type 5 Bartter's syndrome, where there is activating mutation of the calcium sensing receptor in the thick ascending loop of Henle and the distal tubule. In this case this activation of the calcium sensing receptor was triggered by amikacin. This phenomenon has been described with gentamicin though never with amikacin. Recovery of the tubular dysfunction took 15 days following cessation of the offending drug, Amikacin.

Early hypocaloric enteral nutritional supplementation in acute organophosphate poisoning--a prospective randomized trial

INTRODUCTION

Early institution of enteral feeds may be associated with improved outcomes in the critically ill. This study evaluated the effect of hypocaloric enteral nutritional supplementation in acute organophosphate(OP)-poisoned patients requiring invasive mechanical ventilation.

SETTINGS AND DESIGN

Prospective randomized controlled trial conducted in the medical intensive care unit (ICU) of a tertiary care university teaching hospital.

PATIENTS AND METHODS

During a 13-month period, 87 OP-poisoned patients were admitted. Twenty-seven patients who were not ventilated were excluded. Thirty patients were randomized to the control arm and 29 to the intervention arm. The intervention arm received hypocaloric nasogastric feeds within 48 h of intubation whilst the control arm received intravenous fluids. Primary outcome was infectious complications. Secondary outcomes included hospital mortality, duration of ventilation, ICU stay, and hospital stay.

RESULTS

An infectious complication occurred in 14 patients (48%) in the intervention group and 15 patients (50%) in the control group (p = 0.898). Three patients in each group died (p = 0.965). Duration of ventilation (p = 0.19) and ICU stay (p = 0.41) were similar. Duration of hospital stay was shorter in the control group (p = 0.05). Gastric stasis occurred in two patients (6.9%) receiving enteral feeds. Feeding related complications were less frequent than in other published trials.

CONCLUSIONS

In OP-poisoned patients, early hypocaloric enteral feeding was not associated with improvements in clinical endpoints, albeit longer hospital stay was observed in the enterally fed group. Feeding related complications were infrequent. Further studies would help define the status of early enteral feeding in this subset of patients.

Cystic artery pseudoaneurysm after laparoscopic cholecystectomy presenting as haemobilia: a case report

Cystic artery pseudoaneurysm which developed following a cholecystectomy and resulting in upper gastrointestinal bleeding is a rare entity, with only three cases described in the literature. We report the case of a 26-year old man who presented with upper gastrointestinal bleeding approximately three months after laparoscopic cholecystectomy. Emergency abdominal angiogram revealed a cystic artery stump pseudoaneurysm, with no evidence of active contrast extravasation. The pseudoaneurysm was coil embolised and the patient had no further bleeding episodes. In this situation an angiogram and embolisation rather than surgery is the preferred mode of management both in terms of diagnosis and treatment. The presence of a dilated cystic artery stump on angiogram following cholecystectomy is an "ominous sign", even in the absence of active extravasation of contrast.

Acute parent artery dissection as a complication of mesenteric endovascular coil embolisation for pancreatic pseudoaneurysm

We wish to highlight arterial dissection as an unusual complication during endovascular coiling of a pancreatic pseudoaneurysm. Immediate recognition and prompt corrective measures prevented progression of this serious condition. In our patient, angioplasty prevented further propagation of the dissection and preserved coeliac artery patency.

Biotechnology products and European consumers

More than 100 interviews conducted during 1997 with European food manufacturers and retailers, trade associations, government departments, consumer groups, environmental organizations and some individual academic scientists revealed how differences in the perceived attitudes of consumers gave rise to varying approaches by suppliers to the possible introduction of transgenic foods. European consumers generally are not against the pharmaceutical products of biotechnology but are much less willing to accept food and food ingredients, especially when derived from genetically modified plants. Objections are mainly based on fears for the health and safety of the consumer, worries about the possibility of deleterious effects on the environment, and a range of moral and ethical concerns often deriving from a distaste, however expressed, at the concept of interfering with nature. Consumer understanding of the science underlying biotechnology is patchy; in no country does more than a small proportion of the population claim a good grasp. Partly no doubt as a consequence of these attitudes, the introduction of genetically modified foods into Europe has occurred slowly and, during the period of this study, perhaps only in the Netherlands and the UK.

Synthesis and surface activity properties of hydrophobic/hydrophilic peptides

In order to explore the ability of amphiphilic peptides to behave as surface-active agents with emulsifying properties, several short peptides of leucine and glutamine were synthesized with different periodicity, length and hydrophobic characteristics. The stepwise liquid-phase procedure using the N-hydroxysuccinimide ester was deployed in all chain-lengthening steps, and the same procedure was also used subsequently to modify some of the products by introducing a lipophilic moiety such as a palmitoyl residue. The surface-active properties of these products were evaluated by measuring the variation of surface (gamma s) and interfacial (gamma i) tensions and the formation of micelles as a function of concentration in aqueous solution. The alternating sequence (Leu-Gln)n showed good surface activity behaviour, similar to those of recognized surfactants, albeit with no emulsifying function. More hydrophobic compounds, such as lipopeptides, lowered the surface tension of water at concentrations markedly below those usually observed for classical surfactants, and emulsifying properties were observed in all the peptides substituted with lipophilic moieties.

A thirty-year review of maternal mortality in Oklahoma, 1950 through 1979

Oklahoma's Maternal Mortality Committee has been active since 1941. During the 30-year period 1950 through 1979, the committee reviewed in detail 75.9% of the pregnancy-related deaths that occurred in Oklahoma. The maternal mortality ratio in 1950 was 95.1/100,000 live births, and for 1979 it was 8.1/100,000 live births, a decrease of 91.5%. The risk of death from childbearing remained greater for black women than for American Indian or white women throughout the three decades. For American Indian women, the risk of death associated with pregnancy has decreased and is almost equal to the risk for white women. The Maternal Mortality Committee estimated that two thirds of Oklahoma's maternal deaths were preventable. The proportion of deaths judged preventable did not vary substantially during the study period. We conclude that maternal mortality in Oklahoma can be reduced to fewer than three deaths per 100,000 live births. Intensive monitoring and investigation of deaths and their causes by local maternal mortality committees continues to be an important mechanism for obtaining information to assist health workers in the prevention of deaths.

Microbes and oil recovery

Conventional oilfield operations recover, on average, about a third of the crude oil originally present in a reservoir. Improving the yield depends both on overcoming macro- and micro-geological problems and on progressively compensating for the pressure drop in the reservoir as it is depleted of oil. In some cases this may be achieved by treating the oil-bearing matrix with certain chemicals, most commonly surfactants and viscous or insoluble polymers. One effective way of introducing the chemicals is by employing a population of bacteria in the reservoir as synthetic agents. Some of these processes are shown in the Centrespread illustration.

Lymphocyte transformation in lepromatous leprosy: a study of the influence of disease activity and symptom duration

The lymphocyte hyporesponsiveness to M. leprae of patients with active lepromatous leprosy has been well described. This immune defect is less well understood in terms of its time of origin, possible reversibility and specificity. To further examine the persistence and specificity of this abnormality, lymphocyte transformation tests of 93 leprosy patients to lepromin, BCG and PHA were studied. Among lepromatous patients, a decreased response to M. leprae was seen, whether the disease was active or inactive. Decreased responses to BCG were found in lepromatous patients with active disease, but not in those with inactive disease. The duration of patient symptoms was not associated with differences in LTT responses among the active lepromatous patients.

Proline biosynthesis by cell-free extracts of Escherichia coli and potential errors arising from the use of a bioradiological assay procedure

1. The growth of Escherichia coli proline auxotrophs on medium containing L-proline (50 microgram/ml) induces catabolic enzymes. A bioradiological assay system for proline, using proB cells of E. coli, might give erroneous results owing to proline catabolism by the proline auxotrophs on which the assay depends. 2. Differential utilization of proline and 1-pyrroline-5-carboxylate by the proB cells for the synthesis of protein, and failure of the method to distinguish between these two possible products of the proline-biosynthetic enzymes, might also give rise to error. 3. The proline-dependent incorporation of [14C]phenylalanine into the protein of proline-starved proB auxotrophs was to some degree directly influenced by the presence of crude cell extract from E. coli, even though this was not supplied with substrate and cofactors, and could thus not itself synthesize proline. 4. The kinetics of proline biosynthesis by cell-free extracts were linear and biphasic, only the last phase being affected by the concentrations of substrate and extract. This phenomenon is not understood. 5. Proline biosynthesis is inhibited, not only by high concentrations of ATP, but also by aspartate, glycine, alanine and serine, aspartate having the greatest effect. 6. Attempts at complementation in vitro between extracts of proline auxotrophic mutants were not successful, suggesting the possibility that strain X680 (proA) and/or X278 (proB) may be a double mutant. 7. The enzymes of proline biosyntehsis are co-eluted from a column of Bio-Gel A1.5M in a position corresponding to a mol.wt. of 350000. 8. Comparisons between rates of proline biosynthesis in vivo and in vitro were made.

The enzymes of proline biosynthesis in Escherichia coli. Their molecular weights and the problem of enzyme aggregation

1. By using Bio-Gel A1.5M and Sephadex G-150 columns, crude cell-free extracts of Escherichia coli were fractionated to demonstrate the existence of a proline-biosynthetic aggregate. 2. Sephadex G-150 resolves two glutamyl kinases that are inhibited by proline, with mol.wts. of 125000 and 38000, the reactions of which are Mg2+-dependent. The heavier species is more sensitive to inhibition by proline. 3. Gamma-Glutamyl phosphate reductase and 1-pyrroline-5-carboxylate reductase (EC 1.5.1.2) have mol.wts. of approx. 125000 and 190000 respectively, the specific activity of the latter being 5 X 10(3)-fold greater than either of the other two biosynthetic enzymes or of the total pathway in vivo. 4. Bio-Gel A1.5M chromatography gave a single glutamyl kinase of mol.wt. 250000, and the possibility of this being a constituent of an enzyme complex is discussed.

The tentative identification in Escherichia coli of a multienzyme complex with glycolytic activity

Penicillin spheroplasts of Escherichia coli were ruptured osmotically, by freezing and thawing, or mechanically. Differential centrifugation sedimented 20-30% of the glycolytic enzymes without increasing their specific activities. There was, however, evidence of distinct groups of sedimenting enzymes; growth on different carbon sources could influence the distribution. Sucrose gradient studies gave no evidence of enzyme association but provided estimations of the molecular weight of each enzyme which were close to those subsequently observed on gel filtration. Using the determined molecular weight and a literature value for specific activity, the measured activity ratio of the enzymes was compared with that expected from an equimolar mixture. All values agreed within a factor of five, except for hexokinase. The relative roles of hexokinase and phosphotransferase in E. coli are briefly considered. An equimolar multienzyme aggregate of all the enzymes of glycolysis would have a molecular weight of about 1.6 X 10(6). Chromatography on a Biogel column yielded one fraction, corresponding to a molecular weight of 1.6 X 10(6), which contained a proportion of all the glycolytic enzyme studied; the remaining portion of each enzyme activity was eluted from the column at the position expected from its individual molecular weight. The fraction of mol. wt 1 600 000 was tested for complete glycolysis pathway activity and found not to be different from a reconcentrated mixture of the separated enzymes. Both the eluted and the reconstructed systems showed unexpected activity changes at different protein concentrations. The specific radioactivity of pyruvate formed by these systems from [14C]glucose 6-phosphate was reduced by the presence of unlabelled 3-phosphoglycerate, but by less than would have been expected had the latter been able to participate fully in glycolytic activity. This result indicates that these preparations were capable of selectivity compartmenting glycolytic intermediates. Electron microscope investigation of both systems showed large numbers of regular 30 nm diameter particles which, on disruption, appeared to be composed of smaller units: it is possible that these particles may have been aggregates containing glycolytic enzymes. The possible advantages of a glycolytic multienzyme complex are briefly discussed.

Studies on beta-galactoside transport in a Proteus mirabilis merodiploid carrying an Escherichia coli lactose operon

Merodiploid derivatives bearing an F-linked lac operon (i(+), o(+), z(+), y(+), a(+)) from Escherichia coli were prepared from a Proteus mirabilis strain unable to utilize lactose and from a lac deletion strain of E. coli. A suitable growth medium was found in which the episomal element in the P. mirabilis derivative was sufficiently stable to allow induction of the episome-borne lac operon and thus to permit a comparison of the activities and properties of E. coli lac products in the intracellular environments of P. mirabilis and E. coli. In both derivatives the episomal lac operon was shown to be repressed in the absence of inducer. Kinetics of induction with gratuitous inducer (isopropyl-1-thio-beta-d-galactoside) were similar for both beta-galactosidase activity (beta-d-galactoside galactohydrolase, EC 3.4.1.23) and beta-galactoside transport activity in both derivatives, although the ratio of galactoside transport to beta-galactosidase activity was approximately 1.6-fold higher in the E. coli derivative. Comparison of beta-galactosidase and M-protein (lac y gene product)-specific activities indicated coordinate expression of the induced lac operon in both derivatives. Quantitatively, the maximal beta-galactosidase specific activity was two or three times higher for the E. coli derivative. A significant sodium azide inhibition (65% inhibition by 10 mM sodium azide) of lactose permease-mediated transport of o-nitrophenyl-beta-galactoside from an outside region of high concentration to an inside region of very low concentration ("downhill transport") was observed for the P. mirabilis derivative. Identical conditions for the E. coli derivative yielded only about 15% inhibition. Active transport of thiomethyl-beta-galactoside was similar for both derivatives, the major difference being that active transport was more sensitive to azide poisoning in the P. mirabilis derivative. Preliminary examination of the thiomethyl-beta-galactoside derivatives following active transport did not demonstrate the accumulation of a phosphorylated product in either strain but did reveal an unidentified derivative present in the P. mirabilis merodiploid extract which was not detectable in the E. coli merodiploid.

Interactions between metabolic intermediates and beta-galactosidase from Escherichia coli

1. 5-Phosphorylribose 1-pyrophosphate, in the presence of beta-mercaptoethanol, protected beta-galactosidase from heat inactivation. Many other substances, including 3':5'-cyclic-AMP, were without effect. 2. The efficiency of complementation in vitro of beta-galactosidase segments was decreased by 5-phosphorylribose 1-pyrophosphate but not by 3':5'-cyclic-AMP. Neither substance affected the activity of the complete enzyme. 3. Some indications as to the possible identity of the catabolite repression effector are presented.

Adenosine 3':5'-cyclic monophosphate and catabolite repression in Escherichia coli

1. Both permanent and transient catabolite repression of beta-galactosidase synthesis in Escherichia coli are abolished by 5mm-3':5'-cyclic-AMP when elicited by glucose, but not when caused by a mixture of glucose, glucose 6-phosphate, gluconate and casein hydrolysate (casamino acids). 2. Glucose uptake is slightly increased by 3':5'-cyclic-AMP. 3. No significant effects of the nucleotide were found on the synthesis of protein and RNA, either in exponential growth on one substrate, or during a growth shift from glycerol to glycerol plus glucose. 4. Marked changes in the soluble-protein profiles of cells growing in glycerol and glucose were caused by the presence of 3':5'-cyclic-AMP. 5. Measurements of (14)CO(2) release from specifically-labelled glucose showed that 3':5'-cyclic-AMP greatly stimulated glycolytic activity while having a minor depressing effect on the metabolic flow through the pentose phosphate cycle. 6. The concentrations of several metabolic intermediates, particularly fructose 1,6-diphosphate, were greatly affected by the presence of 3':5'-cyclic-AMP. 7. Several metabolites partially relieved glucose repression of beta-galactosidase synthesis in EDTA-treated cells; three out of five of these metabolites reversed the effect more effectively than did 3':5'-cyclic-AMP. 8. The evidence for and against a direct role for 3':5'-cyclic-AMP is discussed. It is concluded that the evidence for indirect action is at least as strong as that for direct action.

Catabolite repression in Escherichia coli. A study of two hypotheses

1. Two hypotheses to account for general catabolite repression of the lactose enzymes in Escherichia coli were tested: the dilution model of Palmer & Moses (1967), and the specific catabolite repressor model of Loomis & Magasanik (1965, 1967). 2. The dilution model predicts that in mutants lacking the i-o regulation system the differential rate of beta-galactosidase synthesis should increase when amino acid-synthesizing enzymes are repressed by the presence of amino acids in the medium. It also predicts that with such mutants the total absence of P(i) from the medium should not result in the complete cessation of beta-galactosidase synthesis that is observed with wild-type cells. 3. Neither prediction was confirmed experimentally, and it is concluded that this model cannot explain catabolite repression. 4. The specific repressor hypothesis depends on the properties of a strain of E. coli carrying the CR(-) mutation. It requires both that cells of this genotype should be totally resistant to general catabolite repression and that this resistance should be specific for the lactose enzymes. 5. In fact the synthesis of beta-galactosidase by CR(-) cells, though showing resistance to catabolite repression by growth on glucose, was found to be repressed in several other circumstances. 6. Two other inducible enzymes, l-tryptophanase and d-serine deaminase, also showed resistance to repression by glucose in CR(-) cells. 7. It is concluded that this model, too, does not account for general catabolite repression. 8. Strains carrying deletions at either end of the lactose operon that extend into the structural genes of the operon continue to exhibit catabolite repression. 9. These experiments appear to eliminate the possibility that catabolite repression operates at the level of DNA transcription, and suggest that repression affects instead the translation of messenger RNA into protein.