Two libraries enriched in murine X chromosome material have been constructed in the X vector NM 1149 from flowsorted chromosomes. Inserts of unique genomic sequence DNA were purified and their X chromosome specificity characterised by hybridisation to a panel of somatic cell hybrid lines. Of the first five such X chromosome-specific probes characterised, all detect restriction fragment length polymorphisms (RFLPs) between inbred mouse laboratory strains such as C57BL/6 and BALB/c and the SPE/Pas mouse strain established from a wild Mus spretus mouse, when their DNAs are digested with the restriction enzyme TaqI. Taking advantage of these RFLPs, all five probes have been localised on the X chromosome using an interspecific backcross between the B6CBARI and SPE/Pas mouse strains segregating the X chromosome markers hypoxanthine phosphoribosyl transferase (Hprt) and Tabby (Ta). Three of the probes map to the region between the centromere and Hprt, and two distal to Ta. Since such X-specific sequence probes detect RFLPs between M. spretus and M. musculus domesticus DNAs with high frequency, a large panel of well localised probes should soon be available for studies of biological problems associated with the X chromosome which can best be approached using the murine species.

Five probes localizing to the Xq26-Xqter region of the human X chromosome have been genetically mapped on the mouse X chromosome using an interspecific cross involving Mus spretus to a contiguous region lying proximally to the Tabby (Ta) locus. Pedigree and recombinational analysis establish the marker order as being Hprt-FIXcll-G6PD-St14-1. The size of this contiguous region is such that the X-linked muscular dystrophy (mdx) mouse mutation probably maps within this segment. This in turn suggests that it is highly improbable that the mouse mdx locus represents a model for Duchenne muscular dystrophy (DMD). It is, however, compatible with the idea that this mutation may correspond in man to Emery Dreifuss muscular dystrophy. The high frequency of restriction fragment length polymorphisms found in this interspecific system for all the human cross-reacting probes examined up until now, using only a limited number of restriction enzymes, suggests that the Mus spretus mapping system may be of great potential value for establishing the linkage relationships existing in man when conserved chromosomal regions are concerned and human/mouse cross-reacting probes are available or can be obtained.In view of the large number of hereditary diseases showing X chromosomal-linked inheritance, considerable effort has been expanded both in generating flanking molecular markers for use in prenatal diagnosis and in establishing an accurate map of the human X chromosome (1). More than 200 such X chromosome loci have been regionally localized using a combination of in situ hybridization, deletion mapping analysis, and, to a lesser extent, family studies (1).Progress has been made in the establishment of an accurate detailed genetic map of the whole human X chromosome by the identification of molecular probes that are capable of either detecting polymorphisms in a series of overlapping family panels, thus allowing the analysis of a series of nested three-point crosses (2), or being used for multipoint analysis (3). However, such efforts are hindered by the relative paucity of probes identifying restriction fragment length polymorphisms (RFLPs) occurring with high frequencies in human populations that are capable of being used in such extensive family studies. One approach to improving this situation involves the isolation of probes recognizing hypervariable mini-satellite sequences (4) so that RFLPs can be identified that are present at a high frequency in human families.A second possible approach involves the use of other experimental systems such as the mouse in which genetic analysis can be facilitated. For such an approach to be valid and to possess advantages over mapping in man itself, the following four basic conditions must be met: (i) Crossreacting molecular probes and/or conserved phenotypic traits must exist. (ii) Linkage relationships must be maintained between the two species for the chromosome in question, at least over small genetic distances. (iii) At least some basic information as to the chromosomal breakpoint...

A panel of five hybrid cell lines containing mouse X chromosomes with various deletions has been obtained by fusing splenocytes from male mice carrying one of a series of reciprocal X-autosome translocations with the azaguanine-resistant Chinese hamster cell line CH3g. These hybrids have been extensively characterized by using the allozymes hypoxanthine/guanine phosphoribosyltransferase (encoded by the Hprt locus) and a-galactosidase (Ags) and a series of 11 X-chromosome-specific DNA probes whose localization had been previously established by linkage studies. Such studies have established the genetic breakpoints of the T(X;12)13RI and T(X;2)14RI X-autosome translocations on the X chromosome and provided additional information as to the X-chromosome genetic breakpoints of the T(X;16)16H, T(X;4)7RI, and T(X;7)6RI translocations. The data establish clearly that both the T(X;4)7RI and T(X;12)13RI X-chromosome breakpoints are proximal to Hprt, the breakpoint of the former being more centromeric, lying as it does in the 9-centimorgan interval between the ornithine transcarbamoylase (Otc) and DXPas7 (M2C) loci. Similarly, it is now clear that the T(X;16)16H X-autosome translocation breakpoint lies distal to the DXPas8 (St14-1) locus, narrowing the X-chromosome breakpoint down to a region flanked proximally by this marker and representing, as expected from previous data, the distal quarter of the Hprt-Ta subchromosomal span. These five hybrid cell lines provide, with the previously characterized EBS4 hybrid cell line, a nested series of seven mapping intervals distributed along the length of the mouse X chromosome. Their characterization not only allows further correlation of the genetic and cytological X-chromosome maps but also should permit the rapid identification of DNA probes specific for particular regions of the mouse X chromosome.While studies on the human genome have progressed rapidly due to our ability both to clone human DNA fragments and to assign such fragments to specific chromosomal regions, analogous studies on the mouse have until recently made less headway.Isolation of chromosome-specific DNA fragments has been hampered by the problems encountered in constructing somatic cell hybrids containing single mouse chromosomes and the difficulty in obtaining high enrichment for specific mouse chromosomes by flow sorting (1). The problems associated with isolating chromosome-specific probes have been compounded by the limited opportunities for obtaining either chromosome or subchromosomal localization. This is due to the small number of hybrid panels available (2), the almost complete absence of murine cell lines containing duplications or deletions for particular chromosomal regions, and, last but not least, the limited amounts of allelic variation found between inbred mouse strains due to the small number of progenitor mice having contributed to their establishment (3).Recent studies have shown that cloned mouse DNA fragments specific for a given chromosome can be generated relatively easily either...

Major advances in our knowledge of the genetic organization of the mouse X chromosome have been obtained by the use of interspecific crosses involving Mus spretus-derived strains. This system has been used to study sequences detected by three probes 80Y/B, 302Y/B and 371Y/B isolated from a mouse Y-chromosome library which have been shown to recognize both male–female common and male–female differential sequences. These patterns are due to the presence of a family of cross-reacting sequences on the mouse X and Y chromosomes. Detailed genetic analysis of the localization of the X-chromosomespecific sequences using both a somatic cell hybrid panel and an interspecific mouse cross has revealed the presence of at least three discrete clusters of loci (X–Y)A, (X–Y)B and (X–Y)C. Two of these clusters, (X–Y)B and (X–Y)C, lie distally on the mouse X chromosome, the other cluster (X–Y)A being situated close to the centromere. In situ hybridization shows a striking symmetry in the localization of the major sequences on both the X and Y chromosomes detected by these probes, hybridization being preferentially localized to a subcentromeric and subtelomeric region on each chromosome. This striking localization symmetry between the X and Y chromosome sequences is discussed in terms of the extensive pairing of the X–Y chromosomes noted during meiosis.