Practice question

Nested Hox activity subdivides proximal-distal axis through overlapping temporal expression domains controlled by global enhancers. In mouse and chick, Hoxa11 and Hoxd11 expressed in region fated as forearm and shank, between Hox9 proximal domain and Hox13 distal domain, specifying zeugopod intermediate segment under balanced RA and FGF inputs and Cyp26b1. Fate maps combined with Cre-mediated knockouts demonstrate Hox11 inactivation deletes radius ulna and tibia fibula. Stylopod marked by Hox9, autopod by Hox13, digits overlapping Hox13 plus Hox12. Therefore Hoxa11 expression identifies zeugopod segment specifically and coordinates growth and patterning.