Ectodermal dysplasias comprise over 150 syndromes of unknown pathogenesis. X–linked anhidrotic ectodermal dysplasia (EDA) is characterized by abnormal hair, teeth and sweat glands. We now describe the positional cloning of the gene mutated in EDA. Two exons, separated by a 200–kilobase intron, encode a predicted 135–residue transmembrane protein. The gene is disrupted in six patients with X; autosome translocations or submicroscopic deletions; nine patients had point mutations. The gene is expressed in keratinocytes, hair follicles, and sweat glands, and in other adult and fetal tissues. The predicted EDA protein may belong to a novel class with a role in epithelial–mesenchymal signalling.