In the seven years since the first human gene was cloned, several hundred coding sequences and many more random single copy DNA sequences have been isolated. Many of these show restriction fragment length polymorphisms (RFLPs) and can be used as genetic markers in inheritance studies. RFLPs enable the construction of complete linkage maps of individual human chromosomes which can then be used as a mapping resource for other genes and disease loci. The isolation of chromosome specific sequences has been greatly facilitated by the purification of human chromosomes by flow cytometry. Sub-localisation of the polymorphic DNA probes along the chromosome can be achieved using in situ hybridisation or rodent/human hybrid cell lines. There are now more than one hundred DNA probes assigned to the human X chromosome and a preliminary genetic map suggests that the chromosome is at least 200 cm long. Some of these DNA sequences have been shown to be linked to disease loci such as Duchenne and Becker muscular dystrophy, X-linked mental retardation and retinitis pigmentosa.