Achondroplasia(ACH) is the most common genetic form of dwarfism. This disorder is inherited as an autosomal dominant trait, although the majority of cases are sporadic. A gene for ACH was recently localized to 4~ 16.3 by linkage analyses. The ACH candidate region includes the gene encoding fibroblast growth factor receptor 3 (FGFR3), which was originally considered as a candidate for the Huntington’s disease gene. DNA studies revealed point mutations in the FGFRS gene in ACH heterozygotes and homozygotes. The mutation on 16 of the 16 ACH-affected chromosomes was the same, a GA transition, at nucleotide 1136 of the cDNA. The mutation on the only ACH-affected chromosome 4 without the GA transition at nucleotide 1136 had a GC transversion at this same position. Both mutations result in the substitution of an arginine residue for a glycine at position 360 of the mature protein, which is in the transmembrane domain of FGFRI.