MTMR1 is a lipid phosphatase belonging to the myotubularin family that catalyzes dephosphorylation of phosphatidylinositol 3-phosphate (PI(3)P) to phosphatidylinositol, and can also convert phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2) to phosphatidylinositol 5-phosphate 12. Structurally, MTMR1 contains conserved pleckstrin homology (PH)-GRAM and protein tyrosine phosphatase (PTP) domains, with substrate specificity determined by strictly conserved residues in the C(X)5R motif 2. The gene arose through duplication from MTM1, the causative gene in X-linked myotubular myopathy, and is located 20 kb distal to MTM1 on Xq28 3. MTMR1 plays a role in muscle differentiation and myogenesis, with a muscle-specific isoform that is induced during myogenesis in vitro and in vivo 4. In myotonic dystrophy type 1 (DM1) and type 2 (DM2), MTMR1 alternative splicing is aberrantly regulated, correlating with impaired muscle fiber maturation and abnormal CUG-binding protein activity 54. While MTMR2 can partially compensate for myotubularin loss in X-linked myotubular myopathy therapy, MTMR1 cannot 6, suggesting functional specialization within the myotubularin family.