Influenza virus type A matrix protein, also known as M1, is composed of a 252 amino acid sequence and is type-specific in influenza viruses. It is located inside the viral lipid envelope and plays a key role in virus assembly and replication. M1 can be isolated from particles by removing the envelope with detergents and reducing the pH to 4.0. The monoclonal antibody FluAc has been shown to specifically recognize type A influenza virus M1 in virus infected cell culture.