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X-linked myotubular myopathy (MTM) is a severe neuromuscular disease of infancy caused by mutations of  MTM1 , which encodes the phosphoinositide lipid phosphatase, myotubularin. The  Mtm1  knockout (KO) mouse has a severe phenotype and its short lifespan (8 weeks) makes it a challenge to use as a model in the testing of certain preclinical therapeutics. Many MTM patients succumb early in life, but some have a more favorable prognosis. We used human genotype–phenotype correlation data to develop a myotubularin-deficient mouse model with a less severe phenotype than is seen in  Mtm1  KO mice. We modeled the human c.205C&gt;T point mutation in  Mtm1  exon 4, which is predicted to introduce the p.R69C missense change in myotubularin. Hemizygous male  Mtm1  p.R69C mice develop early muscle atrophy prior to the onset of weakness at 2 months. The median survival period is 66 weeks. Histopathology shows small myofibers with centrally placed nuclei. Myotubularin protein is undetectably low because the introduced c.205C&gt;T base change induced exon 4 skipping in most mRNAs, leading to premature termination of myotubularin translation. Some full-length  Mtm1  mRNA bearing the mutation is present, which provides enough myotubularin activity to account for the relatively mild phenotype, as  Mtm1  KO and  Mtm1  p.R69C mice have similar muscle phosphatidylinositol 3-phosphate levels. These data explain the basis for phenotypic variability among human patients with  MTM1  p.R69C mutations and establish the  Mtm1  p.R69C mouse as a valuable model for the disease, as its less severe phenotype will expand the scope of testable preclinical therapies.

Modeling the human MTM1 p.R69C mutation in murine Mtm1 results in exon 4 skipping and a less severe myotubular myopathy phenotype