Sequencing studies of candidate genes involved in related neuromuscular or neurodegenerative diseases have identified rare variants in VCP and SQSTM1. Proteomic studies of rimmed vacuoles in IBM and subsequent genetic analyses of candidate genes identified rare missense variants in FYCO1. Complex, large-scale mitochondrial deletions in cytochrome c oxidase-deficient muscle fibres expand our understanding of mitochondrial abnormalities in IBM. The gene discussed is FYCO1; the disease is neurodegenerative disease.