SHH and popliteal pterygium syndrome: Like other genes that are expressed in the ectoderm of the developing face such as SHH and BMP4 [22,23], high IRF6 expression was also detected in the apical ectodermal ridge of the limb buds, which is consistent with the presence of limb hypoplasia or agenesis of digits, syndactyly, as well as valgus or varus deformities of the feet seen in Popliteal pterygium syndrome [24].