Practice question

Zeugopod segment comprising radius-ulna in forelimb and tibia-fibula in hindlimb depends on group 11 Hox paralogs, especially HOXD11, HOXA11, HOXC11. These genes expressed in intermediate limb bud mesenchyme during middle phase of Hox colinear activation. Double knockout Hoxa11/Hoxd11 mice exhibit severely shortened, fused zeugopod elements while stylopod and autopod partially preserved, indicating specific requirement. They regulate proliferation of chondroprogenitors, Fgf10 maintenance period, and integration of SHH timing. Distinct enhancers sensitive to FGF duration control intermediate expression window, establishing zeugopod identity.