The Phase 1 study will enroll two cohorts of 3 XLHED-affected adults each, for a total of 6 subjects. The size and scope of the study design are consistent with a drug development program in an ultra-rare disease and supported by the absence of safety concerns in a GLP non-human primate toxicology study. All subjects must meet entry criteria including documentation of an EDA mutation. Mirroring the conditions of use anticipated for the XLHED-affected newborn study, the adult cohort study incorporates a multiple-dose regimen associated with maximal efficacy in the newborn canine model. Primary outcome measures will be safety, tolerability, immunogenicity and pharmacokinetics (PK). While current data does not support a likelihood of clinical benefit for the XLHED-affected adult subjects, assays of pharmacodynamic/biologic activity are incorporated into the study design as exploratory objectives.