IRF6 and van der Woude syndrome: Consequently, many IRF6 and GRHL3 variants, leading to haploinsufficiency (12) and dominant-negative effects (13), respectively, have been identified in individuals affected by either non-syndromic cleft lip/palate (CLP) or Van der Woude Syndrome (VWS) and Popliteal Pterygium Syndrome, two autosomal dominant conditions characterized by CLP (13–16).