This is a pre-Phase 1 prospective, non interventional clinical assessment study to evaluate XLMTM subjects aged 3 years and younger. Many of these clinically relevant measures have not yet been routinely assessed in this population and may provide important insight on the natural history of XLMTM and for future evaluation of potential therapies.

Inclusion Criteria:

• Subject has a diagnosis of XLMTM resulting from a confirmed mutation in the MTM1 gene
• Subject is male
• Subject is aged less than 4 years
• Subject requires some mechanical ventilatory support (eg, ranging from 24 hours per day full time mechanical ventilation, to noninvasive support such as continuous positive airway pressure [CPAP] or bilevel positive airway pressure [BiPAP] during sleeping hours)
• Access to subject's medical records
• Signed informed consent by the parent(s) or legally authorized representative(s) (LAR) (when applicable)
• Subject and parent(s) or LAR are willing and able to comply with study visits and study procedures

Exclusion Criteria:

• Subject is participating in an interventional study designed to treat XLMTM
• Subject born <35 weeks gestation who is still not to term as per corrected age
• Subject has a clinically important condition, or life-threatening disease other than XLMTM, in the opinion of the investigator
• Subject has received pyridostigmine or any medication to treat XLMTM within 30 days of enrollment

XLMTM subjects aged 3 years and younger.