Practice question

Autopod encompassing carpals, tarsals, metapodials and digits is specified by distal Hox genes HOXA13 and HOXD13 activated during late phase after Hox11 downregulation. Hoxa13 deficient mice display hypoplastic carpals, absent distal condensations, fused digits. These transcription factors regulate EphA7, Sost, and BMP antagonists to establish hand-plate and digit periodicity. Enhancer topology at HoxA/D clusters switches from proximal to distal regulation via 5' enhancers. Therefore HOXA13 serves as definitive transcription factor necessary for autopod identity, distinguishing hand-foot from more proximal arm-leg segments.