Autosomal recessive congenital myopathy-2B (CMYO2B) is a disorder of the skeletal muscle characterized by severe hypotonia with lack of spontaneous movements and respiratory insufficiency, usually leading to death in infancy or early childhood (Agrawal et al., 2004). However, longer survival has also been reported, likely due to the type of mutation and extent of its impact (O'Grady et al., 2015). Mutations in the ACTA1 gene can cause a range of skeletal muscle diseases. About 90% of patients with ACTA1 mutations carry heterozygous mutations, usually de novo (CMYO2A; 161800), whereas 10% of patients carry biallelic ACTA1 mutations (CMYO2B) (Nowak et al., 2007). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000). [from OMIM]