SUMMARY: Transcription factors of the T-box family are required both for early cell-fate decisions, such as those necessary for formation of the basic vertebrate body plan, and for differentiation and organogenesis. When mutated, T-box genes give dramatic phenotypes in mouse and zebrafish, and they have been implicated both in fundamentals of limb patterning and in a number of human congenital malformations such as Holt-Oram, ulnar-mammary and DiGeorge syndromes, as well as being amplified in a subset of cancers. Genes encoding members of the T-box family have recently been shown to comprise approximately 0.1% of genomes as diverse as those of nematodes and humans and have been identified in a wide variety of animals from ctenophores (comb jellies) to mammals; they are, however, completely absent from genomes from other organisms (such as the model plant Arabidopsis thaliana).