Function: Lipid phosphatase which dephosphorylates phosphatidylinositol 3-monophosphate (PI3P) and phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2). Has also been shown to dephosphorylate phosphotyrosine- and phosphoserine-containing peptides. Negatively regulates EGFR degradation through regulation of EGFR trafficking from the late endosome to the lysosome. Plays a role in vacuolar formation and morphology. Regulates desmin intermediate filament assembly and architecture. Plays a role in mitochondrial morphology and positioning. Required for skeletal muscle maintenance but not for myogenesis. Disease: Defects in MTM1 are the cause of centronuclear myopathy X-linked (XCNM) [MIM:310400]; also known as X-linked myotubular myopathy (XLMTM) or myotubular myopathy type 1 (MTM1). Centronuclear myopathies are congenital muscle disorders characterized by progressive muscular weakness and wasting involving mainly limb girdle, trunk, and neck muscles. It may also affect distal muscles. Weakness may be present during childhood or adolescence or may not become evident until the third decade of life. Ptosis is a frequent clinical feature. The most prominent histopathologic features include high frequency of centrally located nuclei in muscle fibers not secondary to regeneration, radial arrangement of sarcoplasmic strands around the central nuclei, and predominance and hypotrophy of type 1 fibers. Similarity: Belongs to the protein-tyrosine phosphatase family. Non-receptor class myotubularin subfamily. Contains 1 GRAM domain. Contains 1 myotubularin phosphatase domain. Domain: The GRAM domain mediates binding to PI(3,5)P2 and, with lower affinity, to other phosphoinositides.