This is an observational, multicentre, international study over a 2-year follow-up period.

The aim of this study is to understand how XLHED affects the lives of young male patients and their families over time. By studying the natural course of the disease and its impact, the study could improve the understanding of the challenges faced by these patients and their families.

XLHED is a rare genetic condition that affects more severely males. The main symptoms are missing or reduced ability to sweat, leading to a risk of dangerous overheating, as well as few or no teeth and sparse hair. This condition can significantly impact the daily lives of patients and their families.

Given the rarity of the disease and the purely descriptive purposes of the study, all eligible patients may be included over a period of approximately 12 months. It is expected to include between 20 and 30 male patients over one year of enrolment in France and Germany.

Statistical analysis: will be descriptive with no hypothesis tested. Questionnaires will be completed by the child's parent at inclusion and at 1 and 2 years after the inclusion data

Inclusion Criteria:

• Boy
• Age at inclusion: from birth to the day before the 11th birthday
• XLHED disease that has been diagnosed by:
• genetic testing or
• symptoms (sweating ability, teeth and hair impairment) and genetic diagnosis of the mother

Exclusion Criteria:

• Any previous treatment with ER004 or participation in a clinical trial testing ER004
• Testing for XLHED disease with a negative result

National expert centres specialized in XLHED in France and Germany