Practice question

Proximal limb stylopod encompassing humerus and femur is specified by early Hox9 paralog group including HOXA9, HOXB9, HOXC9, HOXD9 under influence of proximal retinoic acid and transcription factors Meis1 and Meis2. These Hox genes expressed in proximal bud mesenchyme during initial wave, regulating chondrogenic condensation size and survival. Knockout of Hox9 cluster reduces stylopod length while zeugopod and autopod preserved. Temporal model suggests short duration of FGF exposure favors proximal fate where Hox9 remains active. Hence HOXA9 serves as essential determinant for stylopod formation providing proximal-most positional code.