Many vertebrate homeo box-containing genes have been identified on the basis of their sequence similarity with Drosophila developmental genes. Members of the Dlx gene family contain a homeobox that is related to that of Distal-less (Dll), a gene expressed in the head and limbs of the developing fruit fly. The Distal-less (Dlx) family of genes comprises at least 6 different members, DLX1-DLX6. Trichodentoosseous syndrome (TDO), an autosomal dominant condition, has been correlated with DLX3 gene mutation. This gene is located in a tail-to-tail configuration with another member of the gene family on the long arm of chromosome 17. Mutations in this gene have been associated with the autosomal dominant conditions trichodentoosseous syndrome and amelogenesis imperfecta with taurodontism.