ACTC1 encodes cardiac muscle alpha actin. This isoform differs from the alpha actin that is expressed in skeletal muscle, ACTA1. Alpha cardiac actin is the major protein of the thin filament in cardiac sarcomeres, which are responsible for muscle contraction and generation of force to support the pump function of the heart. Actin is a dynamic structure that can adapt two states of flexibility, with the greatest difference between the states occurring as a result of movement within sub-domain 2. Myosin binding increases the flexibility of actin, and cross-linking studies have shown that myosin subfragment-1 binds to actin amino acid residues 48-67 within actin sub-domain 2, which may account for this effect. It has been suggested that the ACTC1 gene has a role during development. Experiments in chick embryos found an association between ACTC1 knockdown and a reduction in the atrial septa. Polymorphisms in ACTC1 have been linked to Dilated Cardiomyopathy in a small number of Japanese patients. Further studies in patients from South Africa found no association. The E101K missense mutation has been associated with Hypertrophic Cardiomyopathy and Left Ventricular Noncompaction. Another mutation has in the ACTC1 gene has been associated with atrial septal defects.