Achondroplasia, the most common cause of chondrodysplasia in man, is characterized by short-limbed dwarfism, macrocephaly, and dysplasia of metaphyses of the tubular bones. Recently, mutations in the gene encoding fibroblast growth factor receptor-3 (FGFR-3) have been found in patients with achondroplasia. All mutations so far reported had occurred at codon 380, resulting in the substitution of an arginine for a glycine in the transmembrane domain of the predicted protein. We have examined the transmembrane domain of the FGFR-3 gene in seven Japanese patients with achondroplasia. Of the six cases that were sporadic, all carried a mutation in codon 380; the single familial case bore a novel mutation of a G-to-T transition at codon 375, which resulted in substitution of a cysteine for a glycine.