X-linked hypohidrotic ectodermal dysplasia (XLHED; #MIM 305100), a rare hereditary disease characterized by missing or dysplastic skin appendages and teeth, is caused by deficiency of the signaling protein ectodysplasin A1 (EDA1) during early development (Mikkola and Thesleff, 2003). The gene discussed is EDA; the disease is hypohidrotic ectodermal dysplasia.