MYH1 Myopathy: Immune-Mediated Myositis / Nonexertional Rhabdomyolysis by Amy Young, Dr. Carrie Finno, Dr. Stephanie Valberg, and Dr. Rebecca Bellone August 05, 2019 MYHM describes two muscle diseases caused by a mutation in the MYH1 gene; immune mediated myositis and nonexertional rhabdomyolysis.