ACTA1 and myopathy: Mosaicism occurs when a genetic variant is present and/or expressed in only a subset of an organism’s cells (Biesecker and Spinner, 2013), and has been described in several types of myopathy including those related to aberrations in MYH7 (Bader et al., 2022), LMNA (Wang et al., 2023), RYR1 (Estévez-Arias et al., 2024), TPM2 (Tasca et al., 2013), ACTA1 (Miyatake et al., 2014; Lehtokari et al., 2024), and collagen VI-related proteins (Armaroli et al., 2015; Donkervoort et al., 2015).