Which homeobox transcription factor is necessary for stylopod formation?
Proximal limb segments follow Hox clock and retinoic acid environment influencing chromatin accessibility. Hox9 paralog group including Hoxa9 Hoxb9 Hoxc9 Hoxd9 is expressed earliest and most proximally, marking stylopod progenitors that will form humerus and femur under high RA and Meis1-2 induction. Knockdown reduces proximal elements and shifts boundaries distally. Hox11 marks zeugopod, Hox12-13 mark autopod and digits under FGF dominance. Hox9 synergizes with Meis and Pbx cofactors to activate proximal patterning programs. Thus stylopod formation critically depends on Hox9 function interpreting positional information.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Hox9 specification of stylopod humerus/femur.