The human X chromosome contains many disease loci, but only a small number of X-linked genes have been cloned and characterized. One approach to finding genes in genomic DNA uses partial sequencing of random cDNAs to develop {open_quotes}expressed sequence tags{close_quotes} (ESTs). Many authors have recently reported chromosomal localization of such ESTs using hybrid panels. Twenty ESTs specific for the X chromosome have been localized to defined regions with somatic cell hybrids, and 12 of them have been physically linked to markers that detect polymorphisms. One of these ESTs, EST02087, was physically linked in a 650-kb contig to the GLA (α-galactosidase) gene involved in Fabry disease. A comparison of this contig with a 7.5-Mb YAC contig indicated that this gene is also within 250 kb of the src-like protein-tyrosine kinase BTK (X-linked agammaglobulinemia protein-tyrosine kinase) gene in Xq21.3. 14 refs., 1 fig.