HTLV-3 is similar to STLV-3 (Simian T-lymphotropic virus 3). Multiple strains have been identified. It expresses gag, pol, and env, among other proteins. By analogy to other retroviruses, the major envelope glycoprotein, gpl20, of human T-lymphotropic virus type III (HTLV-Ill) is a probable target for neutralizing antibody.