(A) Mouse embryo with developing pharyngeal arches (PA). PA1 (red) gives rise to the muscles of mastication and mandible. PA2 (orange) gives rise to the muscles of facial expression and hyoid bone. PA3 (yellow) gives rise to the greater horn and lower body of the hyoid bone. PA4 (green) gives rise to the thyroid and cricoid cartilage. (B) In the absence of Hoxa2, Runx2 activation will occur leading to bone formation. In the presence of Hoxa2, Runx2 will be suppressed limiting bone formation. (C) The expression of Hoxa2 increases in the caudal direction of the pharyngeal arches with PA1 not having expression of Hoxa2, while PA4 possesses a high level of Hoxa2 expression. Analogously, the expression of Runx2 decreases in the caudal direction of the pharyngeal arches with PA1 having the greatest expression of Runx2, while PA4 possesses a low level of Runx2 expression.