X-linked HED is the most common form of HED and is caused by mutations in EDA. Autosomal HED is caused by mutations in EDAR or EDARADD. Currently, over 100 different mutations in the EDA gene are known, while only ~20 and 4 causative mutations have been found in EDAR and EDARADD, respectively [7]. The gene discussed is EDAR; the disease is hypohidrotic ectodermal dysplasia.