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T he composition of skeletal muscle, in terms of the relative number of slow- and fast-twitch fibers, is tightly regulated to enable an organism to respond and adapt to changing physical demands. The phosphatase calcineurin and its downstream targets, transcription factors of the nuclear factor of activated T cells (NFAT) family, play a critical role in this process by promoting the formation of slow-twitch, oxidative fibers. Calcineurin binds to calsarcins, a family of striated muscle–specific proteins of the sarcomeric Z-disc. We show here that mice deficient in calsarcin-2, which is expressed exclusively by fast-twitch muscle and encoded by the myozenin 1 (Myoz1) gene, have substantially reduced body weight and fast-twitch muscle mass in the absence of an overt myopathic phenotype. Additionally, Myoz1 KO mice displayed markedly improved performance and enhanced running distances in exercise studies. Analysis of fiber type composition of calsarcin-2–deficient skeletal muscles showed a switch toward slow-twitch, oxidative fibers. Reporter assays in cultured myoblasts indicated an inhibitory role for calsarcin-2 on calcineurin, and Myoz1 KO mice exhibited both an excess of NFAT activity and an increase in expression of regulator of calcineurin 1-4 (RCAN1-4), indicating enhanced calcineurin signaling in vivo. Taken together, these results suggest that calsarcin-2 modulates exercise performance in vivo through regulation of calcineurin/NFAT activity and subsequent alteration of the fiber type composition of skeletal muscle.