These considerations notwithstanding, control of dorsoventral trunk pattern by Tbx15 shares certain features with control of dorsoventral limb patterning by Lmx1b, a LIM domain transcription factor that acts downstream of Wnt7a and En1 (Riddle et al. 1995; Vogel et al. 1995; Cygan et al. 1997; Logan et al. 1997; Loomis et al. 1998; Chen and Johnson 2002). Both Tbx15 and Lmx1b act autonomously in mesenchymal cells to promote a dorsal identity, yet have expression domains that do not correspond to cell lineage compartments in the flank (Tbx15) or the limb (Lmx1b) (Altabef et al. 1997; Michaud et al. 1997). In the case of Lmx1b, its expression in the distal limb depends on Wnt7a produced in the overlying dorsal ectoderm (Riddle et al. 1995; Cygan et al. 1997; Loomis et al. 1998). Wnt7a, in turn, is restricted to dorsal ectoderm by En1 in the ventral ectoderm (Loomis et al. 1996; Cygan et al. 1997; Logan et al. 1997), whose expression marks a lineage boundary coincident with the dorsoventral midline of the apical ectodermal ridge (Altabef et al. 1997; Michaud et al. 1997; Kimmel et al. 2000). As described above, En1 or Wnt7a mutations have not been reported to affect patterns of hair-color distribution (C. Loomis, personal communication; Parr and McMahon 1995; Loomis et al. 1996). However, the essential theme that ectodermal lineage compartments control the fate of underlying mesenchyme in developing limbs may apply to the trunk as well as the limb. The mammary glands also develop at a stereotyped dorsoventral position and depend on epithelial–mesenchymal interactions. However, the number and apparent position of the mammary glands are normal in deH/deH animals, indicating the existence of additional mechanisms that control dorsoventral patterning in the trunk as well as in the limbs.