Congenital myopathy-2C (CMYP2C) is an autosomal dominant disorder of the skeletal muscle characterized by severe congenital weakness usually resulting in death from respiratory failure in the first year or so of life. Patients present at birth with hypotonia, lack of antigravity movements, poor head control, and difficulties feeding or breathing, often requiring tube-feeding and mechanical ventilation. Decreased fetal movements may be observed in some cases. Of the patients with congenital myopathy caused by mutation in the ACTA1 gene, about 90% carry heterozygous mutations that are usually de novo and cause the severe infantile phenotype. Some patients with heterozygous mutations have a more typical and milder disease course with delayed motor development and proximal muscle weakness, but are able to achieve independent ambulation (CMYP2A; 161800). The severity of the disease most likely depends on the detrimental effect of the mutation, although there are probably additional modifying factors (Ryan et al., 2001; Laing et al., 2009; Sanoudou and Beggs, 2001; Agrawal et al., 2004; Nowak et al., 2013; Sewry et al., 2019; Laitila and Wallgren-Pettersson, 2021). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (117000).