A Clinical Assessment Study in X-Linked Myotubular Myopathy (XLMTM) Subjects

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