Named for the presence of a DNA-binding domain first identified in the mouse Brachyury gene (haploinsufficiency causes a short tail), T box–containing genes have been identified as developmental regulators in a wide spectrum of tissues and multicellular organisms (reviewed in Papaioannou 2001). The Tbx15 subfamily, which also includes Tbx18 and Tbx22, is likely to have arisen during early chordate evolution since there is a single gene in amphioxus but no obvious homolog in the fly genome (Ruvinsky et al. 2000). Consistent with this relationship, the three genes are expressed in partially overlapping patterns that include anterior somites (Tbx18 and Tbx22), limb mesenchyme (Tbx15 and Tbx18), and craniofacial mesenchyme (all three genes, Tbx15 more broadly than Tbx18 or Tbx22) (Agulnik et al. 1998; Kraus et al. 2001; Braybrook et al. 2002; Bush et al. 2002; Herr et al. 2003). These observations suggest that an ancestral gene for Tbx15, Tbx18, and Tbx22 may have been important for craniofacial development in cephalochordates, with acquisition of additional expression patterns and developmental functions in the limb and the trunk during early vertebrate evolution. Expression of Tbx18 and Tbx22 has not been reported in embryonic flank mesenchyme, which suggests that Tbx15 is the only family member involved in establishing the dorsoventral identity of the trunk. However, it would not be surprising to find some degree of functional redundancy in animals mutated for two or three of the subfamily members in other body regions, particularly the limbs and the head. For example, mutations in Tbx22 cause the human syndrome X-linked cleft palate and ankyloglossia (Braybrook et al. 2001). Despite high levels of Tbx22 expression in periocular embryonic mesenchyme (Braybrook et al. 2002; Bush et al. 2002; Herr et al. 2003), the condition does not affect the eye, perhaps because residual activity is provided by Tbx15 in the same region.