The proposed natural history study will enroll male and female patients, ages 36 months and younger, who have a diagnosis of XLHED based on genetic testing and who have not received an investigational study drug. The study protocol will include collection of all relevant medical history and documentation of clinical outcomes using age-appropriate, minimally invasive technologies. Data will be collected both retrospectively, back to pregnancy assessments that may be available, and prospectively through age 5 yrs.

Important to the development and regulatory approval of therapies for XLHED will be the collection of data on the clinical history and prospective health of those affected by XLHED. The proposed natural history study will enroll male and female patients, ages 36 months and younger, who have a diagnosis of XLHED based on genetic testing and who have not received an investigational study drug. The study protocol will include collection of all relevant medical history and documentation of clinical outcomes using age-appropriate, minimally invasive technologies. Data will be collected both retrospectively, back to pregnancy assessments that may be available, and prospectively through age 5 yrs. Genotype-phenotype correlations in XLHED, based on well-documented health records and prospective assessments on genetically-confirmed individuals, may now provide new and clinically-predictive information for the benefit of patients, families, health care providers and clinical investigators designing trials for therapeutic interventions.

Inclusion Criteria:

Subjects must meet all of the following criteria to be enrolled in this study:

1. Confirmed genetic diagnosis of XLHED
2. Written informed consent of both parents (if reasonably available)

Exclusion Criteria:

Subjects who meet any of the following criteria cannot be enrolled in this study:

1. Medically-significant complications or congenital anomalies outside of those considered to be associated with the diagnosis or status of XLHED
2. Having received an investigational study drug prior to enrollment. For subjects less than 6 months of age, the mother cannot have taken an investigational drug during her pregnancy.
3. Known hypersensitivity to pilocarpine or pilocarpine-like muscarinic agonists
4. Presence of pacemakers

Those with the condition of XLHED up to age 36 months