Cellular hybrids were obtained from a t(X;12) identified in a female patient with hypohidrotic ectodermal dysplasia (EDA). This rearrangement had the same Xq13.1 cytogenetic breakpoint as a t(X;9) found in a previously observed EDA patient. A comparative analysis of these two rearrangements with nine probes was performed at the molecular level. These probes could define three subregions: three are proximal, two are distal, and four are between the two breakpoints. These last probes should prove useful for cloning the gene.