HA-K63R expressing mice display smaller type I fibers. TA muscles from both wild-type (WT) and transgenic (Tg) mice were cryosectioned and immunostained for (A, D) type I, (B, E) type IIA, or (C, F) type IIB. (G) The caliber of positive fibers was measured using ImageScope software (Aperio) and categorized into three groups. The proportion of the fibers falling within the groups was quantified. At least 100 fibers were measured from three independent animals. Results showed a smaller proportion of large type I fibers and more small fibers in transgenic animals (* P<0.05 for Tg versus WT Myosin Heavy Chain type I; t-test comparison).