A Long-term Follow-up Study in Male Participants With X-linked Myotubular Myopathy Treated With ASP2957

X-linked myotubular myopathy (XLMTM) is a rare and serious condition present at birth where the muscles do not work properly. There are currently no approved therapies for XLMTM. The protein myotubularin is needed for muscle development, movement and breathing. A gene called MTM1 tells the body to make myotubularin. XLMTM is caused by changes, or mutations, in the MTM1 gene. Changes in the MTM1 gene cause low or no levels of myotubularin to be made, so the muscles do not work properly. Gene therapy is a way of getting a healthy copy of a gene into the body. This allows the body's cells to make a normal protein that may reduce disease symptoms. Researchers have developed ASP2957 to get a healthy MTM1 gene into the body. This could help improve muscle development and function in young children with XLMTM. A single dose of ASP2957 has previously been given to young boys in an earlier study, but health authorities require up to a 5-year follow-up of the boys. This study is a long-term follow-up study of the young boys with XLMTM who previously received ASP2957 in an earlier study. They won't receive ASP2957 in this long-term study. The main aims of the study is recording medical problems that happen during this study after receiving ASP2957 (in the earlier study), and whether they were caused by ASP2957 or not. This may include abnormal results from blood tests, medical examinations and imaging to check heart health. They may continue to take medicines that lower their immune system from an earlier study. These help to avoid a reaction to ASP2957.