The Evolutionary Consequences of Transposon-Related Pericentromer Expansion in Melon

Transposable elements (TEs) are a major driver of plant genome evolution. A part from being a rich source of new genes and regulatory sequences, TEs can also affect plant genome evolution by modifying genome size and shaping chromosome structure. TEs tend to concentrate in heterochromatic pericentromeric regions and their proliferation may expand these regions. Here, we show that after the split of melon and cucumber, TEs have expanded the pericentromeric regions of melon chromosomes that, probably as a consequence, show a very low recombination frequency. In contrast, TEs have not proliferated to a high extent in cucumber, which has small TE-dense pericentromeric regions and shows a relatively constant recombination rate along chromosomes. These differences in chromosome structure also translate in differences in gene nucleotide diversity. Although gene nucleotide diversity is essentially constant along cucumber chromosomes, melon chromosomes show a bimodal pattern of genetic variability, with a gene-poor region where variability is negatively correlated with gene density. Interestingly, genes are not homogeneously distributed in melon, and the high variable low-recombining pericentromeric regions show a higher concentration of melon-specific genes whereas genes shared with cucumber and other plants are essentially found in gene-rich chromosomal arms. The results presented here suggest that melon pericentromeric regions may allow gene sequences to evolve more freely than in other chromosomal compartments which may allow new ORFs to arise and eventually be selected. These results show that TEs can drastically change the structure of chromosomes creating different chromosomal compartments imposing different constraints for gene evolution.

The Physcomitrella patens chromosome-scale assembly reveals moss genome structure and evolution

The draft genome of the moss model, Physcomitrella patens, comprised approximately 2000 unordered scaffolds. In order to enable analyses of genome structure and evolution we generated a chromosome-scale genome assembly using genetic linkage as well as (end) sequencing of long DNA fragments. We find that 57% of the genome comprises transposable elements (TEs), some of which may be actively transposing during the life cycle. Unlike in flowering plant genomes, gene- and TE-rich regions show an overall even distribution along the chromosomes. However, the chromosomes are mono-centric with peaks of a class of Copia elements potentially coinciding with centromeres. Gene body methylation is evident in 5.7% of the protein-coding genes, typically coinciding with low GC and low expression. Some giant virus insertions are transcriptionally active and might protect gametes from viral infection via siRNA mediated silencing. Structure-based detection methods show that the genome evolved via two rounds of whole genome duplications (WGDs), apparently common in mosses but not in liverworts and hornworts. Several hundred genes are present in colinear regions conserved since the last common ancestor of plants. These syntenic regions are enriched for functions related to plant-specific cell growth and tissue organization. The P. patens genome lacks the TE-rich pericentromeric and gene-rich distal regions typical for most flowering plant genomes. More non-seed plant genomes are needed to unravel how plant genomes evolve, and to understand whether the P. patens genome structure is typical for mosses or bryophytes.

Highly efficient gene tagging in the bryophyte Physcomitrella patens using the tobacco (Nicotiana tabacum) Tnt1 retrotransposon

Because of its highly efficient homologous recombination, the moss Physcomitrella patens is a model organism particularly suited for reverse genetics, but this inherent characteristic limits forward genetic approaches. Here, we show that the tobacco (Nicotiana tabacum) retrotransposon Tnt1 efficiently transposes in P. patens, being the first retrotransposon from a vascular plant reported to transpose in a bryophyte. Tnt1 has a remarkable preference for insertion into genic regions, which makes it particularly suited for gene mutation. In order to stabilize Tnt1 insertions and make it easier to select for insertional mutants, we have developed a two-component system where a mini-Tnt1 with a retrotransposition selectable marker can only transpose when Tnt1 proteins are co-expressed from a separate expression unit. We present a new tool with which to produce insertional mutants in P. patens in a rapid and straightforward manner that complements the existing molecular and genetic toolkit for this model species.

Transposon Insertions, Structural Variations, and SNPs Contribute to the Evolution of the Melon Genome

The availability of extensive databases of crop genome sequences should allow analysis of crop variability at an unprecedented scale, which should have an important impact in plant breeding. However, up to now the analysis of genetic variability at the whole-genome scale has been mainly restricted to single nucleotide polymorphisms (SNPs). This is a strong limitation as structural variation (SV) and transposon insertion polymorphisms are frequent in plant species and have had an important mutational role in crop domestication and breeding. Here, we present the first comprehensive analysis of melon genetic diversity, which includes a detailed analysis of SNPs, SV, and transposon insertion polymorphisms. The variability found among seven melon varieties representing the species diversity and including wild accessions and highly breed lines, is relatively high due in part to the marked divergence of some lineages. The diversity is distributed nonuniformly across the genome, being lower at the extremes of the chromosomes and higher in the pericentromeric regions, which is compatible with the effect of purifying selection and recombination forces over functional regions. Additionally, this variability is greatly reduced among elite varieties, probably due to selection during breeding. We have found some chromosomal regions showing a high differentiation of the elite varieties versus the rest, which could be considered as strongly selected candidate regions. Our data also suggest that transposons and SV may be at the origin of an important fraction of the variability in melon, which highlights the importance of analyzing all types of genetic variability to understand crop genome evolution.

Extensive amplification of the E2F transcription factor binding sites by transposons during evolution of Brassica species

Transposable elements (TEs) are major players in genome evolution. The effects of their movement vary from gene knockouts to more subtle effects such as changes in gene expression. It has recently been shown that TEs may contain transcription factor binding sites (TFBSs), and it has been proposed that they may rewire new genes into existing transcriptional networks. However, little is known about the dynamics of this process and its effect on transcription factor binding. Here we show that TEs have extensively amplified the number of sequences that match the E2F TFBS during Brassica speciation, and, as a result, as many as 85% of the sequences that fit the E2F TFBS consensus are within TEs in some Brassica species. We show that these sequences found within TEs bind E2Fa in vivo, which indicates a direct effect of these TEs on E2F-mediated gene regulation. Our results suggest that the TEs located close to genes may directly participate in gene promoters, whereas those located far from genes may have an indirect effect by diluting the effective amount of E2F protein able to bind to its cognate promoters. These results illustrate an extreme case of the effect of TEs in TFBS evolution, and suggest a singular way by which they affect host genes by modulating essential transcriptional networks.

Dietary abscisic acid ameliorates influenza-virus-associated disease and pulmonary immunopathology through a PPARγ-dependent mechanism

The anti-inflammatory phytohormone abscisic acid (ABA) modulates immune and inflammatory responses in mouse models of colitis and obesity. ABA has been identified as a ligand of lanthionine synthetase C-like 2, a novel therapeutic target upstream of the peroxisome proliferator-activated receptor γ (PPARγ) pathway. The goal of this study was to investigate the immune modulatory mechanisms underlying the anti-inflammatory efficacy of ABA against influenza-associated pulmonary inflammation. Wild-type (WT) and conditional knockout mice with defective PPARγ expression in lung epithelial and hematopoietic cells (cKO) treated orally with or without ABA (100 mg/kg diet) were challenged with influenza A/Udorn (H3N2) to assess ABA's impact in disease, lung lesions and gene expression. Dietary ABA ameliorated disease activity and lung inflammatory pathology, accelerated recovery and increased survival in WT mice. ABA suppressed leukocyte infiltration and monocyte chemotactic protein 1 mRNA expression in WT mice through PPARγ since this effect was abrogated in cKO mice. ABA ameliorated disease when administered therapeutically on the same day of the infection to WT but not mice lacking PPARγ in myeloid cells. We also show that ABA's greater impact is between days 7 and 10 postchallenge when it regulates the expression of genes involved in resolution, like 5-lipoxygenase and other members of the 5-lipoxygenase pathway. Furthermore, ABA significantly increased the expression of the immunoregulatory cytokine interleukin-10 in WT mice. Our results show that ABA, given preventively or therapeutically, ameliorates influenza-virus-induced pathology by activating PPARγ in pulmonary immune cells, suppressing initial proinflammatory responses and promoting resolution.

Immunoregulatory actions of epithelial cell PPAR gamma at the colonic mucosa of mice with experimental inflammatory bowel disease

BACKGROUND

Peroxisome proliferator-activated receptors are nuclear receptors highly expressed in intestinal epithelial cells (IEC) and immune cells within the gut mucosa and are implicated in modulating inflammation and immune responses. The objective of this study was to investigate the effect of targeted deletion of PPAR gamma in IEC on progression of experimental inflammatory bowel disease (IBD).

METHODOLOGY/PRINCIPAL FINDINGS

In the first phase, PPAR gamma flfl; Villin Cre- (VC-) and PPAR gamma flfl; Villin Cre+ (VC+) mice in a mixed FVB/C57BL/6 background were challenged with 2.5% dextran sodium sulfate (DSS) in drinking water for 0, 2, or 7 days. VC+ mice express a transgenic recombinase under the control of the Villin-Cre promoter that causes an IEC-specific deletion of PPAR gamma. In the second phase, we generated VC- and VC+ mice in a C57BL/6 background that were challenged with 2.5% DSS. Mice were scored on disease severity both clinically and histopathologically. Flow cytometry was used to phenotypically characterize lymphocyte and macrophage populations in blood, spleen and mesenteric lymph nodes. Global gene expression analysis was profiled using Affymetrix microarrays. The IEC-specific deficiency of PPAR gamma in mice with a mixed background worsened colonic inflammatory lesions, but had no effect on disease activity (DAI) or weight loss. In contrast, the IEC-specific PPAR gamma null mice in C57BL/6 background exhibited more severe inflammatory lesions, DAI and weight loss in comparison to their littermates expressing PPAR gamma in IEC. Global gene expression profiling revealed significantly down-regulated expression of lysosomal pathway genes and flow cytometry results demonstrated suppressed production of IL-10 by CD4+ T cells in mesenteric lymph nodes (MLN) of IEC-specific PPAR gamma null mice.

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate that adequate expression of PPAR gamma in IEC is required for the regulation of mucosal immune responses and prevention of experimental IBD, possibly by modulation of lysosomal and antigen presentation pathways.

[Intimate partner violence in Spain]

OBJECTIVE

To estimate mortality and the incidence of formal complaints of intimate partner violence (IPV) by age groups and Spanish provinces.

METHODS

We performed a descriptive, epidemiological study of murders by intimate partners registered in the database of the Divorced and Separated Women's Federation (1998-2001) and of formal complaints of IPV in The Home Affairs database (1997-2000). The percentage of increase in formal complaints and deaths and the individual risk of dying from IPV and of reporting this crime was calculated through the simple accumulative method, specific mortality rates by age, average rates by provinces, and specific incidence rates of formal complaints of IPV by provinces. Finally, a simple regression analysis was performed to assess the relation between mortality and the incidence of formal complaints of IPV.

RESULTS

Between the first and the last year of the study, formal complaints increased by 27% and murders increased by 49%. In Spain, during the period analyzed, one out of every 100,000 women was murdered by an intimate partner and one out of every 200 women reported IPV. Women aged 22-41 years and those aged 82-86 years presented the highest rates of mortality from IPV. Mortality from IPV and the incidence of formal complaints of IPV showed no clear geographical pattern. No correlation between mortality and the incidence of formal complaints of IPV by provinces was found (corrected r2 = 0.016).

CONCLUSIONS

IPV is an increasing public health problem in Spain. There seems to be a bimodal curve by age in mortality from IPV, but no geographical patterns in mortality and the incidence of formal complaints of IPV or between the provinces with the highest incidence of formal complains and those with highest mortality were observed.

Nutritional regulation of glucose-6-phosphatase gene expression in liver of the gilthead sea bream (Sparus aurata)

To examine the role of glucose-6-phosphatase (G6Pase) in glucose homeostasis in the diabetes-like experimental model of carnivorous fish, we analysed postprandial variations and the effect of starvation, ration size and diet composition on the regulation of G6Pase expression at the enzyme activity and mRNA level in the liver of gilthead sea bream (Sparus aurata). G6Pase expression increased in long-term starved or energy-restricted fish. In contrast to data reported for other fish species, short-term regulation of G6Pase expression was found in regularly fed S. aurata. G6Pase mRNA levels were lowest between 4 and 15 h after food intake, whereas minimal enzyme activity was observed 10-15 h postprandially. Alterations of plasma glucose levels affect G6Pase in mammals. However, the carbohydrate content of the diet did not affect hepatic expression of G6Pase in S. aurata, suggesting that a different molecular mechanism is involved in the control of G6Pase expression in fish. Although G6Pase was unaffected, high-carbohydrate low-protein diets increased glucokinase (GK) expression and thus allowed a metabolic adaptation favouring glycolysis over gluconeogenesis. Interestingly, only the nutritional conditions that promoted variations in the blood glucose levels resulted in changes in the hepatic expression of G6Pase. These findings indicate a concerted regulation of G6Pase and GK expression and suggest that the direction and rate of the glucose-glucose-6-phosphate substrate cycle flux is finely regulated in the liver of S. aurata, challenging the role attributed to deficient regulation of G6Pase or GK expression in the low ability of carnivorous fish to metabolize glucose.

Molecular spectra of HPRT deletion mutations in circulating T-lymphocytes in Fanconi anemia patients

The principal cellular feature of Fanconi anemia (FA), an inherited cancer prone disorder, is a high level of chromosomal breakage, amplified after treatment with crosslinking agents. Three of the eight genes involved in FA have been cloned: FANCA, FANCC and FANCG. However, their biological functions remain unknown. We previously observed an excessive production of deletions at the HPRT locus in FA lymphoblasts belonging to the relatively rare complementation group D(1) and an increased frequency of glycophorin A (GPA) variants in erythrocytes derived from FA patients (2). In thi study, we examined the molecular nature of 31 HPRT mutations formed in vivo in circulating T-lymphocytes isolated from 9 FA male patients. The results show that in all FA patients investigated the deletions are by far the most prevalent mutational event in contrast to age matched healthy donors, in which point mutations predominate. The complementation group in the FA patients examined in the present study has not yet been defined. However, knowing that mutations in the FANCA and FANCC gene are found to be involved in at least 70% of the FA patients, it can be expected that the excessive production of deletions is a general feature of the FA phenotype. In addition, the spectrum of HPRT deletions observed in FA patients differs from that of healthy children: there is a high frequency of 3'-terminal deletions and a strikingly low proportion of V(D)J mediated events. Based on previous findings, a decreased fidelity of coding V(D)J joint formation (3) and an inaccurate repair of specific DNA double strand breaks via Non-Homologous End Joining (4), we propose that FA genes play a role in the control of the fidelity of rejoining of specific DNA ends. Such a defect may explain several basic features of FA, such as chromosomal instability and deletion pronenness.

Coordinated modulation of the fibroblast growth factor dual receptor mechanism during transformation from human colon adenoma to carcinoma

Basic fibroblast growth factor (bFGF) is dependent on heparan sulphate for its ability to activate the cell surface signal transducing receptor. We have investigated the FGF dual receptor mechanism in a novel model of the transformation from human colon adenoma to carcinoma in vitro. Reverse transcription-polymerase chain reaction showed that mRNA for FGF receptors 1 and 2 were expressed in both the adenoma and carcinoma cells whereas immunocytochemistry showed that the expression of the FGF R1 was reduced significantly in the carcinoma cells. We have reported previously that the composition and sequence of human colon adenoma and carcinoma heparan sulphate (HS) differ in a defined and specific manner. The functional significance of these changes was assessed by affinity co-electrophoresis, which showed that the affinity of adenoma HS for bFGF was 10-fold greater than that of the carcinoma HS (Kd 220 nM vs. 2493 nM, respectively). In addition, Northern studies of the expression of syndecan 1 and 4 mRNA showed that proteoglycan core protein expression was reduced significantly in the carcinoma cells. These findings were associated with a reduced biological response to bFGF in the carcinoma cells that could be partially reversed by the addition of exogenous heparin, suggesting that both the proteoglycan and signal transducing receptor control the cells' response to bFGF.

[Evaluation of measurements intended to develop regularity of blood donation]

An increase in the regularity of blood donation is desirable for two main reasons. First, the lower incidence of viral disease markers in regular donors when compared to first donors could help to reduce risks of transmissible diseases. Second, a higher frequency of donation could contribute to a more satisfactory supply of blood products. Two measures implemented with the aim of increasing the regularity of blood donation were evaluated: (i) an increase in the annual frequency of blood collection by mobile teams at collection sites, and (ii) a "next donation document" given to each donor indicating the earliest possible date of the next donation. The regularity index was calculated as the mean number of cellular donations (whole blood and platelet apheresis) per donor per year, over two consecutive periods: 1-7-1993-30-6-1994 (P1) and 1-7-1994-30-6-1995 (P2). The junction of these two periods corresponded to the date of introduction of the "next donation document" and to the time of reinforcement of the mobile collection frequency. First donors in either period were not taken into account in the study. A significant relationship was observed between the annual frequency of mobile collection and the average number of donations per donor (comparison test of two means on large samples, p < 0.001 in all the cases excepted those of stable mobile collection numbers). Thus, in the first period, the average donation rate was the lowest on sites having only one mobile collection per year (M: 1.55, W: 1.38) and the highest on sites with five mobile collections per year (M: 2.05, W: 1.71). These average numbers significantly increased in the second period as compared to the first (M: +4.8%, W: +1.8%; comparison test of two means from paired series, p < 0.001), as did the yearly frequency of mobile blood collections (+9.2%). On the other hand, the "next donation document" was not associated to any change in the regularity index. The results of this study therefore showed an increase in the frequency of visits of mobile collection teams to be the main factor leading to an improvement in the regularity of blood donation. Moreover, this evolution was associated with a decrease in the incidence of viral markers detected at donation.

[Quality control of homologous blood collection activities: 5 years' experience]

Quality control (QC) of blood collection activities for transfusion is a regulatory requirement. The authors report on their experience in this field over the past 5 years. In their institution, this QC is based on both the recording and analyzing of predefined data, as well as the search for an active collaboration from each person involved in these activities. QC of medical selection relies on the assessment of several associated criteria: effectiveness of the information given to blood donors for recruitment, preparation of the medical interview and encouragement to perform regular donations; frequency of donors deferred after the medical interview; frequency of biological abnormalities detected at donation; results of the inquiries into the corresponding medical interviews following adverse transfusion reactions. The quantitative and qualitative evaluation of blood collection permits assessment of the quality of the blood collection program, collection procedures and directly derived blood products. Quality assessment of facilities and equipment is also included in this QC. Results have been improving in recent years, especially regarding medical selection. In particular, an increase in the mean donation rate of donors, a decrease in biological abnormalities detected at donation and an absence of adverse transfusion reactions attributed after inquiry to an inadequate medical interview have been noticed. A decrease in both shortage and outdating of labile blood products likewise indicates an improvement of blood collection planning. However, this QC reveals deficiencies in the information given to donors and a lack of analysis of the data specific to first time donors. In order to further improve the efficiency of QC, these results now require comparison with similar data collected on a nation-wide scale.

[Staff certification for mobile blood collection units]

Training and official acknowledgment of the competence of each staff member are essential to the quality and safety of collected blood products prepared and delivered by a blood transfusion center. A procedure was created to indicate in detail the methods employed to implement such accreditation. Based on individual training according to activity, it defines for each type of activity (secretary, physician, collector, driver) the required theoretical and practical knowledge of his/her position. Accreditation, consisting of assessment of the degree of competence attained in these areas of responsibility, was applied to the members of mobile blood collection teams in 1995. No major deficiency was detected, and this certification was well accepted by the staff. In order to complete this initial accreditation, blood collection abnormalities (inadequate blood volumes, clots or defective welding of tubing) were assessed for each collector individually. Comparison of these abnormalities in qualified nurses and laboratory technicians with a blood collection diploma showed no differences. On the other hand, significantly higher numbers of abnormalities were found in intermittent as compared to regular collectors and in senior as compared to new collectors. The applied corrective measures led to obviation of differences and improvement in performance. In 1996, in the first individual evaluation of medical selection carried out by each physician, discrepancies of one to 20 donors (0.7-14.2%) were observed from one doctor to another in the frequency of elimination of candidates for blood donation after the medical interview. Regular meetings with physicians resulted in reducing these discrepancies to one to 3.1 donors (4.6-14.1%) in 1997. In conclusion, the association of an initial accreditation procedure with an individual follow-up of work quality allowed satisfactory assessment of the training and competence of staff members. This kind of method could be extended to those working in other fields of transfusion medicine.

[Study of in vitro antimicrobial resistance of positive coprocultures to Shigella sp]

BACKGROUND

The gastroenteritis caused by Shigella sp (shigellosis) represents an important cause of morbimortality, especially in children under two years old. It is well known that the proper antimicrobial therapy can improve clinical state and diminish the disease dissemination. Increasing levels of resistance to drugs commonly used in the treatment of shigellosis have been described, and its sensibility has not been recently evaluated in our setting.

OBJECTIVE

To evaluate the Shigella antimicrobial resistance in our setting.

METHODS

A retrospective study was conducted using 106 coprocultures of patients, hospitalized or not, who have been assisted at HCPA and at HPV, during the period of 1994 to 1996, with the sensibility test done through the Kirby-Bauer technique of diffusion discs.

RESULTS

Out of the 106 coprocultures studied, 35 (33%) came from HPV and 71 (67%), from HCPA. Considering the species, 58.8% were S. flexneri, 39.6%, S. sonnei, 0.9%, S. disenteriae and 0.9%, S. boydii. The resistance levels to usually indicated antibiotics were: ampicillin -57.5%, trimethropim-sulfamethoxazol -75.5%, and nalidixic acid -7.5%.

CONCLUSIONS

These results show significant (p<0.001) in vitro antimicrobial resistance to ampicillin and trimethoprim-sulfamethoxazol and low resistance to nalidixic acid among Shigella sp. Thus, nalidixic acid may be an option to treat infections caused by resistant organisms.

Overproduction and purification of an agarase of bacterial origin

The agarase gene from Streptomyces coelicolor has been cloned in the non-producer bacterium Streptomyces lividans under the control of its own set of promoters and under the control of a heterologous promoter that is functional only during exponential growth. The best level of overproduction was obtained when the strain containing the natural gene was cultivated in fed batch with mannitol as carbon source. The protein, with a relative molecular mass of 32 kDa, has been purified following an affinity purification method. Contaminating activities seem to be absent from the purified enzyme preparation that can be used to purify DNA from agarose gels.

Interactions between prolactin and ovarian secretions during mammary tumorigenesis in the mouse

The plasma levels of ovarian steroids and the steroid binding capacity in the cytosol of mammary glands have been determined in the low-mammary-tumor C3H/f (XVII) mice. Implantation of a pituitary under the kidney leads to an increase of all these parameters as well as a high mammary tumor incidence. Simultaneous administration of the trans isomer of broparestrol (TBP) leads to a decrease of all the parameters.

Mammary carcinogenesis in (C3H x RIII)F1 mice which receive the trans isomer of a brominated triphenylethylene

The trans isomer of a bromotriphenylethylene (TBP) inhibits spontaneous mammary carcinogenesis in (C3H x RIII)F1 females, which received a diet containing 20 ppm of the chemical: intact, spayed, and spayed and pituitary implanted animals. Mammary carcinogenesis is inhibited even if the administration of the compound starts late in life. TBP has no activity on mammary carcinogenesis of castrated males implanted with an ovary.

Inhibition of prolactin-induced mammary cancer in C3Hf (XVII) mice with the trans isomer of bromotriphenylethylene

C3Hf (XVII) mice never develop spontaneous mammary tumors. However, the transplantation of an isologous pituitary gland under their kidney capsule is followed by a 10-fold increase in serum and pituitary prolactin content (180 ng/ml and 20 micrograms/mg of tissue, respectively), concomitant with an increase of prolactin receptors in mammary glands. Under these conditions, mammary tumors appear in 90% of the mice. If a racemic brominated triphenylethylene, i.e., broparestrol, is administered, serum and pituitary prolactin decrease rapidly (10 ng/ml and 4 micrograms/mg of tissue, respectively), and prolactin receptors in the mammary gland are markedly reduced. This compound also inhibits the development of normal mammary glands, prevents mammary carcinogenesis, and unexpectedly causes a significant atrophy of the ovaries. Our study confirms that prolactin is a key hormone involved in murine mammary carcinogenesis and that it can act directly on the mammary gland by stimulaing the level of its own receptor.

Dose-effect studies on estrogen induced mammary cancers in mice

Castrated male (C3H X RIII) F1 mice were treated with graded doses of estradiol-17-beta and estrone in a life-span experiment. Estradiol-17-beta incorporated into paraffin pellets was implanted under the skin for continuous resorption. These pellets contained 1 microgram, 2.5 microgram, 5 microgram, 10 microgram or 100 microgram of hormone. Estron was given orally, mixed with the food at 3 daily dosages: 0.06 microgram, 0.6 microgram and 6 microgram. Although the smallest dosages of both hormones induced vaginal estrus in castrated females, they did not produce mammary tumors. The mammary tumor incidence reached progressively almost 100% in response to the increase of the hormonal dosages. The absence of effect of low doses of estrogen in mice is compared with the absence of an excess of breast cancers among women using oral contraceptives.