Median facial clefts in Xenopus laevis: Roles of retinoic acid signaling and homeobox genes

► Retinoic acid signaling is required for upper lip and primary palate formation. ► Retinoic acid signaling regulates expression of homeobox genes msx2 and lhx8. ► Retinoic acid signaling and Msx2/Lhx8 regulate cell proliferation and differentiation. ► Median facial clefts form in embryos deficient in retinoic acid, Lhx8 and Msx2.