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 therapies for this serious condition. The protein myotubularin is needed for muscle development and movement. 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 causes low levels of myotubularin to be made, so the muscles do not work properly. XLMTM may also affect the liver, and in some cases, this can be dangerous and threaten the patient´s life. 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. AT132 is a gene therapy that gets a healthy MTM1 gene into the body to help improve muscle development and function in young children with the disease. AT132 does not treat liver disease, and because of the way the treatment works, it may make liver problems worse. AT132 was the gene therapy treatment given to children who participated in this study and is not available to the public. In this study, AT132 was given to children for the first time. Due to the occurrence of severe complications and fatalities associated with administration of AT132, the study has been stopped and no further participants will be enrolled. The main aim of the study is to check how long young children need machines to support breathing (ventilation support) after AT132. Due to the occurrence of severe complications and fatalities associated with administration of AT132, the study has been stopped and no further participants will be enrolled. This study included children with XLMTM under 5 years old who had breathing problems caused by XLMTM. They couldn't take part if they were born prematurely, recently had surgery, had liver disease or other condition or disease the study doctor thought was medically important. The study did enroll participants with medically significant liver disease. This is an open-label study. This means that young children and their caregivers, and clinic staff know that young children received AT132. This study was designed with 2 parts and is now in a long-term follow-up phase to collect information on the safety and improvements in muscle function in the children who received AT132. In Part 1, small groups of young children were given different doses of AT132, with one group receiving a lower dose and one group receiving a higher dose of AT132. The purpose of giving the two doses was to determine which dose was best for treating the muscle disease. After receiving AT132, a medical panel of experts reviewed each child for safety and for how their muscles responded. AT132 did not demonstrate appropriate safety at either dose. Administration of AT132 was stopped. Children who received AT132 are being monitored for 10 years for safety and to understand how their muscles function over time.