Carrier detection in typical and atypical X-linked agammaglobulinemia

We have recently demonstrated that B cells from obligate carriers of typical X-linked agammaglobulinemia (XLA) exhibit nonrandom X chromosome inactivation. The active X is always the X that does not carry the gene defect. To determine if this were also true in carriers of atypical XLA and to provide carrier detection for all women at risk of being carriers of XLA, we developed a technique that permits analysis of X chromosome inactivation in cells from any woman. This technique combines the production of somatic cell hybrids that selectively retain the active X chromosome with the use of X-linked restriction fragment length polymorphisms that permit the distinction of the two X chromosomes. Three obligate carriers of typical XLA and four women whose sons might be considered to have atypical or sporadic XLA were studied. B cell hybrids from all seven women demonstrated exclusive use a single X as the active X. In addition, B cell hybrids from four of eight women at 25% or 50% risk of being carriers exhibited nonrandom X chromosome inactivation, indicating that these women were also carriers of X-linked forms of hypogammaglobulinemia. These results illustrate a technique that can be used both to help define XLA and to provide carrier detection for all women at risk of being carriers of this disorder.

Nonrandom X chromosome inactivation in B cells from carriers of X chromosome-linked severe combined immunodeficiency

X chromosome-linked severe combined immunodeficiency (XSCID) is characterized by markedly reduced numbers of T cells, the absence of proliferative responses to mitogens, and hypogammaglobulinemia but normal or elevated numbers of B cells. To determine if the failure of the B cells to produce immunoglobulin might be due to expression of the XSCID gene defect in B-lineage cells as well as T cells, we analyzed patterns of X chromosome inactivation in B cells from nine obligate carriers of this disorder. A series of somatic cell hybrids that selectively retained the active X chromosome was produced from Epstein-Barr virus-stimulated B cells from each woman. To distinguish between the two X chromosomes, the hybrids from each woman were analyzed using an X-linked restriction fragment length polymorphism for which the woman in question was heterozygous. In all obligate carriers of XSCID, the B-cell hybrids demonstrated preferential use of a single X chromosome, the nonmutant X, as the active X. To determine if the small number of B-cell hybrids that contained the mutant X were derived from an immature subset of B cells, lymphocytes from three carriers were separated into surface IgM positive and surface IgM negative B cells prior to exposure to Epstein-Barr virus and production of B-cell hybrids. The results demonstrated normal random X chromosome inactivation in B-cell hybrids derived from the less mature surface IgM positive B cells. In contrast, the pattern of X chromosome inactivation in the surface IgM negative B cells, which had undergone further replication and differentiation, was significantly nonrandom in all three experiments [logarithm of odds (lod) score greater than 3.0]. These results suggest that the XSCID gene product has a direct effect on B cells as well as T cells and is required during B-cell maturation.

Assignment of the complement serine protease genes C1r and C1s to chromosome 12 region 12p13

C1r and C1s are distinct, but structurally and functionally similar, serine protease zymogens responsible for the enzymatic activity of the first component of complement (C1). Recent comparisons indicate a significant degree of sequence similarity between C1r and C1s and support the hypothesis that they are related by gene duplication. Complementary DNA probes for human C1r and C1s do not cross-hybridize even at mild stringency conditions and are therefore gene-specific. Using a panel of 25 human-rodent cell hybrids, we have independently assigned the C1r and the C1s genes to chromosome 12. In situ hybridization analyses were consistent with these assignments, showing in addition that both C1r and C1s are located on the short arm of the chromosome in the region p13. These data suggest that the homologous C1r and C1s genes have remained closely linked after duplication of a common ancestor. The C1r and C1s loci also provide useful polymorphic DNA markers for the short arm of chromosome 12.

Genetic mapping of the mouse c-fms proto-oncogene to chromosome 18

Chinese hamster X mouse somatic cell hybrids were analyzed by Southern blot hybridization with a probe specific for the cellular c-fms proto-oncogene. Results demonstrate that Fms, the genetic locus containing this sequence, maps to mouse chromosome 18. Mouse Fms is thus not linked to the same set of genes involved in growth regulation that human FMS is linked to.

p53 in chronic myelogenous leukemia. Study of mechanisms of differential expression

The p53 is a nuclear protein that is associated with normal cellular proliferation and can cooperate with Ha-ras in causing cellular transformation in vitro. Lineage association is known to exist between p53 expression and normal lymphopoiesis, but not myelopoiesis. We studied the expression of p53 using chronic myelogenous leukemia (CML) cell lines, somatic hybrids of these cells, and leukemic cells from CML patients. Lymphoid CML lines expressed both p53 mRNA and protein. We also analyzed p53 synthesis by two B-lymphoid lines from the same CML patient; cells of one line were derived from the neoplastic clone, cells of the other were derived from the normal clone. Both synthesized equal amounts of a phosphorylated p53 protein. None of the myeloid CML lines expressed detectable p53 protein and two of four expressed negligible p53 mRNA. Two other myeloid CML lines and myeloid cells from three of four patients expressed p53 mRNA. These findings suggest that expression of the gene is not regulated normally in CML. Several approaches were pursued to explore the differential expression of p53. Southern blot analyses showed no gross alterations in the p53 gene from cells of either the expressing or the nonexpressing lines. No difference in the pattern of demethylated CpG sites was noted in the region of the p53 gene in cells from K562 (myeloid p53 nonexpressor) and in BV173 (lymphoid p53 expressor). The sites of demethylation clustered in and around the p53 promoter in both cell lines. Somatic hybrids formed between a p53 mRNA nonexpressor myeloid line (K562) and the parental p53 expressor lymphoid lines (Daudi, PUT) produced p53 mRNA and protein, suggesting that p53 is a dominantly expressed protein and that lack of expression in myeloid cells is not mediated by a trans-acting negative regulatory protein. DNA transfection experiments performed using the indicator gene chloramphenicol acetyltransferase attached to promoter sequences of p53 showed that these constructs were equally activated in BV173 (p53 expressor) and K562 (p53 mRNA nonexpressor). The mechanism of p53 regulation in CML remains unclear.

I.29 lymphoma cells express a nonmutated VH gene before and after H chain switch

The I.29 B cell lymphoma consists of IgM+ and IgA+ cells which express the same germ-line VH gene. IgA+ cells of the I.29 lymphoma were derived from the IgM+ cells by a typical H chain switch recombination event. The IgM+ cells can be induced with LPS to undergo H chain switching in culture. It has been proposed that the somatic hypermutation process is activated during H chain switch, since V genes expressed in IgG+ and IgA+ cells have more frequently undergone mutation than those expressed in IgM+ cells. We have investigated this question by sequencing VH genes expressed before and after H chain switch in the I.29 lymphoma. We have also sequenced the germ-line VH gene corresponding to the gene expressed by I.29 cells to determine whether the VH gene expressed in the IgM+ cells had already undergone somatic mutation. Our results indicate that somatic mutation was not activated in the precursor cell for the I.29 lymphoma, nor during isotype switch in I.29 cells. It is possible that cells of the I.29 lymphoma, or their precursor, have not received the signal which induces somatic mutation, or that I.29 cells belong to a subset of B cells that cannot be induced to undergo any (or much) somatic mutation.

Different molecular weight forms of opioid receptors revealed by polyclonal antibodies

Polyclonal antibodies were raised against a purified opioid receptor from bovine brain (Cho, et. al., 1986), and shown to inhibit 3H-diprenorphine binding to this receptor in a dose-dependent fashion. These antibodies were then used to characterize opioid-binding material present in rat brain and in NG108-15 neuroblastoma-glioma hybrid cells. Western blot analysis revealed that the antibodies reacted with a single species of 58,000 molecular weight in rat brain membranes; this closely corresponds in size to the bovine opioid receptor used to raise the antibodies. In contrast, the polyclonal antibodies reacted with a 45,000 molecular weight species in NG108-15 neuroblastoma-glioma hybrid cells; moreover, this band was specifically reduced in NG108-15 cells in which opioid receptors had been down-regulated by incubation with D-ala2-D-leu5-enkephalin for 24 hours. Thus at least two distinct opioid receptor molecules have been identified, which have antigenic similarities.

Chromosomal localization of satellite DNA sequences among 22 species of felids and canids (Carnivora)

In situ hybridization was carried out using cloned satellite DNAs from the domestic cat and domestic dog as probes to metaphase chromosomes from 12 species of felids and 10 species of canids. Autoradiographic silver grains along metaphase chromosomes were counted and analyzed with regard to the mean number of grains per cell in each species, their chromosomal location, and their presence or absence on specific autosomes or sex chromosomes, where known. Among the felids and canids there was a 7.6- and 8.9-fold statistically significant difference, respectively, in the mean number of grains per cell between the species having the minimum and maximum values. Among the felids, most grains occurred on the telomeres of D- and E-group chromosomes, although departures from this general pattern also occurred. For example, the Asian golden cat and the Bornean bay cat showed substantial labeling at the centromeric region of chromosome A1, and a number of species showed some labeling at the short-arm telomeres of B-group chromosomes. Among the canids, about 90% of all grains were located at autosomal centromeres, and grains were absent from the sex chromosomes. Grains are usually distributed at chromosomal locations that stain C-band positive; however, certain C-band-positive regions without grains probably do not contain the particular satellites studied here.

The gene for protein S maps near the centromere of human chromosome 3

Two different mapping approaches were used to determine the human chromosomal location of the gene for protein S. A human protein S cDNA was used as a hybridization probe to analyze a panel of somatic cell hybrids containing different human chromosomes. Cosegregation of protein S-specific DNA restriction fragments with human chromosome 3 was observed. Three cell hybrids containing only a portion of chromosome 3 were analyzed in order to further localize protein S. Based on the somatic cell hybrid analysis, protein S is assigned to a region of chromosome 3 that contains a small part of the long arm and short arm of the chromosome including the centromere (3p21----3q21). In situ hybridization of the protein S cDNA probe to human metaphase chromosomes permitted a precise localization of protein S to the region of chromosome 3 immediately surrounding the centromere (3p11.1----3q11.2). Protein S is the first protein involved in blood coagulation that has been mapped to human chromosome 3.

Chromosomal localization of the human G-CSF gene to 17q11 proximal to the breakpoint of the t(15;17) in acute promyelocytic leukemia

The G-CSF gene encodes a hematopoietic colony-stimulating factor (CSF) that promotes growth, differentiation, and survival of neutrophilic granulocytes. By analysis of somatic cell hybrids and in situ chromosomal hybridization, we localized this gene to human chromosome 17, at bands q11 to q12, the region of the breakpoint on chromosome 17 in the 15;17 translocation [t(15;17)] characteristic of acute promyelocytic leukemia. To determine the position of the G-CSF gene in relation to the breakpoint junctions and to evaluate the possible role of G-CSF in the pathogenesis of promyelocytic leukemia, we applied the techniques of in situ chromosomal hybridization and Southern blot analysis to leukemia cells from eight acute promyelocytic leukemia patients who had a t(15;17). Our results indicate that the G-CSF gene is proximal to the breakpoint of the t(15;17) and that this gene remains on the rearranged chromosome 17. Southern blot analysis using conventional and pulsed-field gel electrophoresis did not reveal rearranged restriction fragments, indicating that no rearrangements had occurred within the G-CSF gene or in surrounding sequences.

A chromosome 11-linked determinant controls fetal globin expression and the fetal-to-adult globin switch

Hybrids formed by fusing mouse erythroleukemia (MEL) cells with human fetal erythroid cells produce human fetal globin, but they switch to adult globin production as culture time advances. To obtain information on the chromosomal assignment of the elements that control gamma-to-beta switching, we analyzed the chromosomal composition of hybrids producing exclusively or predominantly human fetal globin and hybrids producing only adult human globin. No human chromosome was consistently present in hybrids expressing fetal globin and consistently absent in hybrids expressing adult globin. Subcloning experiments demonstrated identical chromosomal compositions in subclones displaying the fetal globin program and those that had switched to expression of the adult globin program. These data indicate that retention of only one human chromosome-i.e., chromosome 11--sufficient for expression of human fetal globin and the subsequent gamma-to-beta switch. The results suggest that the gamma-to-beta switch is controlled either cis to the beta-globin locus or by a trans-acting mechanism, the genes of which reside on human chromosome 11.

Opal suppressor phosphoserine tRNA gene and pseudogene are located on human chromosomes 19 and 22, respectively

An opal suppressor phosphoserine tRNA gene and pseudogene have been isolated from a human DNA library and sequenced (O'Neill, V., Eden, F., Pratt, K., and Hatfield, D. (1985) J. Biol. Chem. 260, 2501-2508). Southern hybridization of human genomic DNA with an opal suppressor tRNA probe suggested that the gene and pseudogene are present in single copy. In this study, we have determined the chromosome location of the human gene and pseudogene by utilizing a 193-base pair fragment encoding the opal suppressor phosphoserine tRNA gene as probe to examine DNAs isolated from human-rodent somatic cell hybrids that have segregated human chromosomes. These studies show that the probe hybridized with two regions in the human genome; one is located on chromosome 19 and the second on chromosome 22. By comparing the restriction sites within these two regions to those previously determined for the human opal suppressor phosphoserine tRNA gene and pseudogene, we tentatively assigned the gene to chromosome 19 and the pseudogene to chromosome 22. These assignments were confirmed by utilizing a 350-base pair fragment which was isolated from the 5'-flanking region of the human gene as probe. This fragment hybridized only to chromosome 19, demonstrating unequivocally that the opal suppressor phosphoserine tRNA gene is located on chromosome 19. The flanking probe hybridized to a single homologous band in hamster and in mouse DNA to which the gene probe also hybridized, demonstrating that the 5'-flanking region of the opal suppressor tRNA gene is conserved in mammals. Restriction analysis of DNAs obtained from the white blood cells of 10 separate individuals demonstrates that the gene is polymorphic. This study provides two additional markers for the human genome and constitutes only the second set of two tRNA genes assigned to human chromosomes.

Mutants of embryonal carcinoma cells defective in the expression of embryoglycan

Embryonal carcinoma cells defective in the expression of developmentally regulated carbohydrate epitope of teratocarcinoma cells (TEC-1) were isolated from mutagenized P19X1 and P19S1801A1 cells by a single-step selection technique using monoclonal antibody TEC-01 conjugated to plant toxin ricin. Three independently isolated mutant cell lines were characterized in detail. Analysis of the expression of the TEC-1 epitope in somatic cell hybrids constructed between wild-type and mutant cells and between two mutant cell types revealed that the mutant phenotypes are recessive and that the mutants belong to, at least, two complementation groups. Each mutant cell line exhibited a unique binding pattern of four monoclonal antibodies and five lectins, and different properties of large glycopeptides were distinguished by Sephadex G-50 column chromatography. The combined data suggest that our mutants identify three genes involved in the synthesis of embryoglycan, one of which appears to be the regulatory or structural gene for fucosyltransferase. One mutant cell line was completely deprived of embryoglycan and several carbohydrate structures typical of early embryonic and embryonal carcinoma cells; however, the cells were similar to parental cells in their morphology, their ability to form aggregates when cultured in suspension, their ability to differentiate into neuron-like cells after treatment with retinoic acid, and their ability to form tumors composed of embryonal carcinoma cells. Thus, embryoglycan is not required for the expression of a number of properties of the embryonal carcinoma phenotype.

Studies on the polyoma tumor-specific transplantation antigen (TSTA): selection and characterization of TSTA-negative segregants from somatic hybrids

Two polyoma-TSTA-negative variants were selected independently from a polyoma fibrosarcoma/Moloney lymphoma somatic hybrid, by repeated passages in polyoma-virus-preimmunized mice. One of the variants had lost all its polyoma DNA, while the other only retained a deleted piece of its integrated polyoma DNA. In contrast to the parental hybrid clone, none of the variants produced detectable amounts of T-antigens. This finding indicates that a detectable expression of the products of the polyoma virus early genome, the T-antigens, is important either directly or indirectly for the expression of TSTA.

Alpha-chain locus of the T-cell antigen receptor is involved in the t(10;14) chromosome translocation of T-cell acute lymphocytic leukemia

Human leukemic T cells carrying a t(10;14)(q24;q11) chromosome translocation were fused with mouse leukemic T cells, and the hybrids were examined for genetic markers of human chromosomes 10 and 14. Hybrids containing the human 10q+ chromosome had the human genes for terminal deoxynucleotidyltransferase that has been mapped at 10q23-q25 and for C alpha [the constant region of TCRA (the alpha-chain locus of the T-cell antigen receptor gene)], but not for V alpha (the variable region of TCRA). Hybrids containing the human 14q- chromosome retained the V alpha genes. Thus the 14q11 breakpoint in the t(10;14) chromosome translocation directly involves TCRA, splitting the locus in a region between the V alpha and the C alpha genes. These results suggest that the translocation of the C alpha locus to a putative cellular protooncogene located proximal to the breakpoint at 10q24, for which we propose the name TCL3, results in its deregulation, leading to T-cell leukemia. Since hybrids with the 10q+ chromosome also retained the human terminal deoxynucleotidyltransferase gene, it is further concluded that the terminal deoxynucleotidyltransferase locus is proximal to the TCL3 gene, at band 10q23-q24.

Cytogenetic and molecular studies on a recombinant human X chromosome: implications for the spreading of X chromosome inactivation

A pericentric inversion of a human X chromosome and a recombinant X chromosome [rec(X)] derived from crossing-over within the inversion was identified in a family. The rec(X) had a duplication of the segment Xq26.3----Xqter and a deletion of Xp22.3----Xpter and was interpreted to be Xqter----Xq26.3::Xp22.3----Xqter. To characterize the rec(X) chromosome, dosage blots were done on genomic DNA from carriers of this rearranged X chromosome using a number of X chromosome probes. Results showed that anonymous sequences from the distal end of the long arm to which probes 4D8, Hx120A, DX13, and St14 bind as well as the locus for glucose-6-phosphate dehydrogenase (G6PD) were duplicated on the rec(X). Mouse-human cell hybrids were constructed that retained the rec(X) in the active or inactive state. Analyses of these hybrid clones for markers from the distal short arm of the X chromosome showed that the rec(X) retained the loci for steroid sulfatase (STS) and the cell surface antigen 12E7 (MIC2); but not the pseudoautosomal sequence 113D. These molecular studies confirm that the rec(X) is a duplication-deficiency chromosome as expected. In the inactive state in cell hybrids, STS and MIC2 (which usually escape X chromosome inactivation) were expressed from the rec(X), whereas G6PD was not. Therefore, in the rec(X) X chromosome inactivation has spread through STS and MIC2 leaving these loci unaffected and has inactivated G6PD in the absence of an inactivation center in the q26.3----qter region of the human X chromosome. The mechanism of spreading of inactivation appears to operate in a sequence-specific fashion. Alternatively, STS and MIC2 may have undergone inactivation initially but could not be maintained in an inactive state.

Chromosomal localization of the gene coding for alpha-subunit of Na+,K+-ATPase in the American mink (Mustela vison)

The gene coding for the alpha-subunit of Na+,K+-ATPase has been localized on chromosome 2 of the American mink (Mustela vison) using the somatic cell hybrids mink-Chinese hamster and pig cDNA clones as hybridization probes.

Human apolipoprotein D gene: gene sequence, chromosome localization, and homology to the alpha 2u-globulin superfamily

The exons and bordering intron nucleotides of the human apolipoprotein D (apo D) gene have been sequenced. The protein-coding portion of the gene is divided into five exons which span approximately 12,000 bp. At least one intron interrupts the 5' untranslated region. The gene has been localized to the p14.2----qter region of human chromosome 3. Apo D shares homology with the alpha 2u-globulin superfamily of genes, including approximately 25% amino acid homology with human retinol-binding protein (RBP). Similarity of intron locations in both apo D and RBP suggests that these two genes derived from a common ancestor.

Assignment of the Ly-6--Ril-1--Sis--H-30--Pol-5/Xmmv-72--Ins-3--Krt-1--Int-1 --Gdc-1 region to mouse chromosome 15

Previous work has demonstrated linkage between Ly-6, H-30, and a locus, Ril-1, that affects susceptibility to radiation-induced leukemia. Results or preliminary linkage analyses suggested further that the cluster might be linked to Ly-11 on the proximal portion of mouse chromosome 2. Using molecular probes to examine somatic cell lines and recombinant inbred and congenic strains of mice, we have re-evaluated these linkage relationships. A cloned genomic DNA fragment derived from a retroviral site has been used to define a novel locus, Pol-5, that is tightly linked to both H-30 and Ril-1 as shown by analysis of the B6.C-H-30c congenic mouse strain. Following the segregation of the Pol-5 mouse-specific DNA fragment in a series of somatic cell hybrids carrying various combinations of mouse chromosomes on a rat or Chinese hamster background mapped Pol-5 to mouse chromosome 15. During the course of these studies, restriction fragment length polymorphisms were defined associated with several loci, including Pol-5, Ly-6, Sis, Ins-3, Krt-1, Int-1, and Gdc-1. Three of these loci, Sis, Int-1, and Gdc-1, have been previously mapped to chromosome 15 by others using somatic cell hybrids or isoenzyme analyses. Following the inheritance of these eight loci in recombinant inbred strains of mice allowed the definition of a linkage group on the chromosome with the order Ly-6--Ril-1--Sis--H-30--Pol-5--Ins-3--Krt-1--Int-1--Gdc-1. Analyses of alleles inherited as passengers in B6.C-H-30c, C3H.B-Ly-6b, and C57BL/6By-Eh/+ congenic mouse strains and in situ hybridization experiments support the above gene order and indicate further that the cluster is located on distal chromosome 15, with Ly-6 and Sis near Eh.

DNA sequence and regional assignment of the human follicle-stimulating hormone beta-subunit gene to the short arm of human chromosome 11

A human genomic DNA fragment in phage lambda containing FSHB, the gene for the beta-subunit of human follicle-stimulating hormone (FSH-beta), was analyzed and the nucleotide sequence of the region of the clone encoding FSH-beta was determined. A subclone of the lambda phage containing 67% of FSH-beta coding sequence was used as hybridization probe to determine the human chromosomal location of FSHB. A panel of mouse-human somatic cell hybrids containing reduced numbers of human chromosomes was screened with the FSHB probe; complete cosegregation of FSHB with human chromosome 11 was observed in all 26 cell hybrids tested. Analysis of a set of cell hybrids containing translocated derivatives of chromosome 11 further localized FSHB to the human chromosome region 11p11.2----11pter. A Hind III restriction fragment length polymorphism (RFLP) detected by another subclone of the lambda phage containing FSHB now provides a genetic marker for this region of the human genome.

Use of restriction enzymes to detect potential gene sequences in mammalian DNA

Only a small proportion of the vertebrate genome codes for proteins. It would therefore be useful if genes, and in particular the sites at which transcription begins, could be identified in libraries of cloned DNA. Since many known vertebrate genes have distinctive sequences (HTF-islands) surrounding their transcription start sites, we wished to be able to select these sequences easily and to find out how diagnostic they are for genes. HTF-islands contain a high density of non-methylated CpG (ref. 2) and can be detected in chromosomal DNA as clustered sites for certain rare-cutting (C-G) restriction enzymes. Identification of islands in chromosomal DNA is aided by methylation which blocks C-G enzyme sites in non-island DNA. This advantage is lost in cloned DNA, where CpG methylation is absent. We have calculated, however, that even in cloned DNA most sites for certain C-G enzymes should occur in HTF-islands. We tested this prediction using the enzyme SacII and found that four out of four sites in separate cosmids from the human X chromosome were located in HTF-islands. Hybridization to Northern blots provided preliminary evidence that three of the islands were associated with genes.

Physical and genetic analysis of cosmids from the vicinity of the cystic fibrosis locus

Cosmid libraries have been constructed from DNA of somatic cell hybrid cell lines, each containing a fragment of human chromosome seven and including sequences closely linked to cystic fibrosis (CF). Cosmids containing human DNA as insert were isolated from the library. Three cosmids, when used as probes to total genomic DNA, detected polymorphic loci, each of which was shown to be in strong linkage disequilibrium with CF. Restriction endonuclease digestion of cosmid clones and use of a new, rapid method of chromosome walking based on competitive hybridisation of cosmid inserts has allowed identification of several groups of overlapping cosmids ("contigs") from the vicinity of CF.

Molecular analysis and chromosomal mapping of amplified genes isolated from a transformed mouse 3T3 cell line

We are exploring the origin and function of amplified DNA sequences associated with double minutes (DMs) in a spontaneously transformed derivative of mouse 3T3 cells. Toward that goal, we have constructed a cDNA library using RNA from these cells and have isolated cDNA clones representing sequences that are amplified and overexpressed in these 3T3-DM cells. From results of Northern- and Southern-blot analyses, we conclude that these cDNAs represent two distinct genes, which we have designated mdm-1 and mdm-2. Using DNAs from a panel of Chinese hamster-mouse somatic cell hybrids together with in situ hybridization protocols for gene mapping studies, we have found that these DM-associated, amplified DNA sequences originate from mouse chromosome 10, region C1-C3. Sequences homologous to mdm-1 and mdm-2 are present in the genomes of several species examined, including that of man.