MBNL1 and myotonic dystrophy type 1: To determine the relationship between myotonia severity and the oxidative state of muscle fibers in DM1 mouse models, we first performed immunofluorescence analysis of gastrocnemius muscles in mice with severe (LR41;Mbnl1−/−), moderate (Mbnl1−/−), mild (LR20b), and no myotonia (WT) using antibodies specific for myosin heavy chain type 1 (MyHC 1), MyHC 2 A, and MyHC 2B.