Ulnar-mammary syndrome is a rare pleiotropic disorder affecting limb, apocrine gland, tooth and genital development. We demonstrate that mutations in human T8X3, a member of the T-box gene family, cause ulnar-mammary syndrome in two families. Each mutation (a single nucleotide deletion and a splice-site mutation) is predicted to cause haploinsufficiency of TBX3, implying that critical levels of this transcription factor are required for morphogenesis of several organs. Limb abnormalities of ulnar-mammary syndrome involve posterior elements. Mutations in TBX5, a related and linked gene, cause anterior limb abnormalities in Holt-Oram syndrome. We suggest that during the evolution of TBX3 and TBX5 from a common ancestral gene, each has acquired specific yet complementary roles in patterning the mammalian upper limb.