Hypohidrotic ectodermal dysplasia (HED) is a complex genetic disorder characterized by lack of sweat glands, sparse hair, and missing or malformed teeth. Inability to sweat may result in episodes of severe hyperthermia and cause sudden infant death. To assess sweat gland function in HED patients, the investigators will first quantify gland pores in a defined area of the palm and then stimulate the glands by pilocarpine followed by sweat collection in a special capillary for volume determination. This will be combined with non-invasive skin conductance measurement prior and subsequent to stimulation of the sympathetic nervous system. The data should provide a basis for genotype-phenotype correlation.