AbstractThe history and the lessons learned from hypohidrotic ectodermal dysplasia (HED) may serve as an example for the unraveling of the cause and pathogenesis of other ectodermal dysplasia syndromes by demonstrating that phenotypically identical syndromes (HED) can be caused by mutations in different genes (EDA, EDAR, EDARADD), that mutations in the same gene (EDA) can lead to different phenotypes (HED and selective tooth agenesis) and that mutations in genes further downstream in the same signaling pathway (NEMO) may modify the phenotype quite profoundly (incontinentia pigmenti (IP) and HED with immunodeficiency). But it also demonstrates that diligent phenotype characterization and classification is extremely helpful in uncovering the underlying genotype. We also present a new mutation in the EDA gene which causes selective tooth agenesis and demonstrates the phenotype variation that can be encountered in the ectodermal dysplasia syndrome (HED) with the highest prevalence worldwide. © 2009 Wiley‐Liss, Inc.