A cardiac and skeletal muscle disorder caused by variation in the gene ACTN2. Cardiac features include but are not limited to cardiac features such as dilated cardiomyopathy, hypertrophic cardiomyopathy, restrictive cardiomyopathy, arrhythmias, left ventricular non-compaction, and left-dominant arrhythmogenic cardiomyopathy. Skeletal features include but are not limited to progressive distal and/or proximal muscle weakness, gait disturbance, muscle atrophy, and elevated creatine kinase.