Rapp-Hodgkin syndrome (RHS) belongs to a group of genetic disorders known as ectodermal dysplasia syndromes. In general, ectodermal dysplasia syndromes are caused by defects in the formation of the ectoderm (the outermost layer of embryonic tissue) during gestation. Ectodermal dysplasia syndromes are multisystem disorders that are characterized by the deficient function or absence of at least two of the following ectodermal structures: skin, teeth, hair, nails, and glands (National Foundation for Ectodermal Dysplasias [NFED], 2001). More than 200 clinically or genetically distinct forms of ectodermal dysplasia have been identified, with classification based on the following considerations: extent, nature, and severity of ectodermal involvement; the presence of associated malformations; and the mode of inheritance.