We have isolated 23 human X chromosome-specific DNA fragments from lambda libraries, prepared from flow-sorted X chromosomes. To increase diagnostic potential for X-linked genetic disorders, including Duchenne muscular dystrophy (DMD), the fragments were tested for restriction fragment length polymorphisms (RFLPs) with six restriction enzymes. All fragments were regionally mapped to segments of the X chromosome with a panel of somatic cell hybrids and with human cell lines carrying unbalanced chromosomal abnormalities. Two of the isolated probes detected a high frequency RFLP. One, 754, maps between Xp11.3 and Xp21 and detects a PstI polymorphism with an allele frequency of 0.38. The other, 782, maps between Xp22.2 and Xp22.3 and reveals an EcoRI polymorphism with an allele frequency of 0.40. According to a pilot linkage study of families at risk for Duchenne muscular dystrophy, 754 gives a maximum Lod score of 7.6 at a recombination fraction of 0.03. Probe 782 lies telomeric to DMD with a maximum Lod score of 2.2 at a recombination fraction of 0.17. Using our X-chromosomal probes and a set of autosomal probes, isolated and examined in an identical way, we found a significantly lower RFLP frequency for the X chromosome as compared to the autosomes.

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From a human-Chinese hamster somatic cell hybrid a clone was derived containing chromosome 13 in duplicate as its only human material. This clone was used to construct a human chromosome 13-specific recombinant DNA-library. Overlapping Sau3AI DNA sequences (11.9-17.2 kb) from the cell hybrid were inserted into the lambda phage vector EMBL4. From eleven recombinants having a human insert thirteen putative unique DNA sequences were isolated and cloned into the plasmid vector pBR329. A human-mouse hybrid containing a human chromosome 13 with a deletion of 13q14 and lacking its undeleted homologue was constructed to be used in a selection procedure for DNA sequences belonging to band q14. Three probes originating from two different phages were assigned to 13q14 because they did not hybridise to DNA from this cell hybrid. One of these 13q14 probes detects a low frequency (2/44) MspI restriction fragment length polymorphism. The probes are now being used for screening a cosmid library to find adjacent polymorphic sequences with a RFLP information content suitable for application in the diagnosis of hereditary retinoblastoma.

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