Achondroplasia (ACH), the most common cause of chondrodysplasia in man (1 in 15,000 live births), is an autosomal dominant condition of unknown origin characterized by short-limbed dwarfism and macrocephaly. Recently, a gene for ACH has been mapped to chromosome 4p16.3. The genetic interval encompassing the disease gene contains a member of the fibroblast growth factor receptor (FGFR) family which is expressed in articular chondrocytes (FGFR,). We report here recurrent missense mutations, in a CpG doublet of the transmembrane domain of the FGFR, protein (G380R) in 17 sporadic cases and 6 unrelated familial forms of ACH and show that the mutant genotype segregates with the disease in these families. Thus, it appears that recurrent mutations of a single amino acid in the transmembrane domain of the FGFR, protein account for all cases (23/23) of achondroplasia in our series.