INCLUSION CRITERIA  Male Patients ages 2 years and older may participate. XLMTM resulting from a confirmed mutation in the Myotubularin 1 (MTM1) gene Patients over 18 years of age and parent(s)/legal guardian(s) of patients <18 years of age must provide written informed consent prior to participating in the study and informed assent will be obtained from minors, or at least 7 years of age when required by regulation. Willing and able to comply with all protocol requirements and procedures.  EXCLUSION CRITERIA  Other disease which may significantly interfere with the assessment of myotubular myopathy (MTM) and is clearly not related to the disease, at the discretion of the qualified investigator. Has undergone surgery or hospitalization < 3 months before starting TAM4MTM (at t = -3 months), or has surgery scheduled during the 18 months of participation in TAM4MTM, which will impede motor assessments in the opinion of the Investigator. Has a history of thromboembolic events Currently enrolled in a treatment study for XLMTM or receiving treatment with an experimental therapy other than pyridostigmine. Treatment with pyridostigmine for < 6 weeks duration (must be greater than 6 weeks to be included in TAM4MTM). Use of concomitant medication known to inhibit CYP2D6 and/or CYP3A4, including clarithromycin, erythromycin, diltiazem, itraconazole, ketoconazole, ritonavir, verapamil, goldenseal and grapefruit, paroxetine, troleandomycin, rifampin, phenobarbital, aminoglutethimide, medroxyprogesterone, amiodarone, haloperidol, indinavir, ritonavir, quinidine, rifampicin, or any selective serotonin reuptake inhibitor (SSRI). Subject has a contraindication to tamoxifen or its ingredients