Lipid phosphatase which efficiently dephosphorylates phosphatidylinositol 3-phosphate (PtdIns3P) and PtdIns(3,5)P2; inactive toward PtdIns4P, PtdIns(3,4)P2, PtdIns(4,5)P2 and PtdIns(3,4,5)P3. Found in reticular structures and plasma membrane ruffles. Concentrated near the nucleus (By similarity). In C2C12 cell line, increased expression during myotube formation and differentiation in culture. Belongs to the protein-tyrosine phosphatase family. Non-receptor class myotubularin subfamily.