Affected males of X-linked anhidrotic ectodermal dysplasia show hypotrichosis, abnormal teeth and absent sweat glands. Some of the patients reported by Halperin and Curtis showed mental defect also, but this is not an invariable feature. Ectodysplasin A (EDA) is a trimeric type II membrane protein that co-localizes with cytoskeletal structures at the lateral and apical surfaces of cells. EDA is expressed in hair follicles and in the epidermis of adult skin. The sequence of the longest isoform includes an interrupted collagenous domain of 19 Gly-X-Y repeats and a motif conserved in the tumor necrosis factor (TNF)-related ligand family. EDA is a member of the TNF-related ligand family involved in the early epithelial-mesenchymal interaction that regulates ectodermal appendage formation. Similar to other members of collagenous membrane proteins and members of TNF-related ligands, EDA is a type II membrane protein which forms trimers.