X-linked hypohidrotic ectodermal dysplasia (XLHED; OMIM 305100) is characterized by sparse hair, abnormal teeth and decreased sweating as a result of abnormal development of the sweat glands. Mutations in the ED1 gene, which encodes ectodysplasin-A (EDA), are responsible for XLHED. Ectodysplasin-A, a ligand for the EDA receptor, plays an important role in epidermal morphogenesis. We identified ED1 mutations including three novel mutations by sequencing genomic DNAs from eight unrelated Japanese XLHED families. Data from all reported mutations revealed that codon 156 in the furin subdomain is the most frequent site of change in EDA.