This idea might be consistent with the fact that knockout mice lacking EGFR ligands have defects in specific tissues, such as impaired ductal outgrowth of mammary glands in Areg KO mice12, 13, wavy hair formation in Tgfα KO mice55, heart valve malformation in Hb-egf KO mice56 and dermatitis in Ereg KO mice57. The gene discussed is EREG; the disease is skin disorder.