MTM1 and X-linked myotubular myopathy: XLMTM is due to the lack of myotubularin (MTM1), a lipid phosphatase that dephosphorylates phosphatidylinositol 3-phosphate (PtdIns(3)P) and PtdIns(3,5)P2 into PtdIns and PtdIns(5)P7–9.