Antibodies to the antigen Actin generally refer to those that are reactive against a family of highly conserved proteins that make up the filaments of the cell cytoskeleton as well as key components of muscle tissue. It exists as both monomer (G-actin) or as a filamentous polymer (F-actin) and is encoded in humans by six genes: ACTA1, ACTA2, ACTB, ACTC1, ACTG1, and ACTG2. Medical professionals screen for the presence of actin auto-antibodies (often referred to as smooth muscle antibodies or SMA) in the blood is a standard test for diseases such as autoimmune Hepatitis. But researchers use these antibodies as tools; antibodies to the beta-actin form of the protein in particular are commonly used as a loading control in Western Blots.