(A) The Kaplan–Meier survival analysis of Mtm1 p.R69C (n = 57) and wild-type littermate males (n = 54). Mtm1 p.R69C animals have a median survival of 66 weeks (53.8 ± 2.9 weeks), log-rank P < 0.001. Laryngeal muscles in 3-month-old Mtm1 p.R69C (B) and wild-type littermate (C) showing small fibers with central nuclei in the former (B, arrowheads). Bar = 50 μm. (D) Body mass of Mtm1 p.R69C animals and wild-type littermates over 12 months (n = 12 for each group). At 1 month, there is no significant difference in body mass between Mtm1 p.R69C (13.1 ± 3.2 g) and wild-type (14.3 ± 1.2 g) animals; however, the difference at 2 months (Mtm1 p.R69C: 15.8 ± 1.3 versus wild-type: 21.2 ± 2.7 g) is significant (*P < 0.001), and this coincides with the onset of weakness. Mtm1 p.R69C body mass is ∼50% that of wild-type animals up to 12 months. Abbreviations: KI, Mtm1 p.R69C; WT, wild-type.