Human MTM1, a 603 amino-acid protein, is mutated in myotubular myopathy. The largely related protein hMTMR2 is found mutated in a recessive form of Charcot-Marie-tooth neuropathy. Myotubularin is primarily a lipid phosphatase that acts on hosphatidylinositol 3-monophosphate and is involved in the regulation of the phosphatidylinositol 3-kinase (PI 3-kinase) pathway and membrane trafficking. Wildtype myotubularin can directly dephosphorylate PI 3-P and PI 4-P in vitro. Thus, it decreases PI 3-P levels by down-regulating PI 3-K activity and by facilitating the degradation of PI 3-P.