The human T-lymphotropic virus, human T-cell lymphotropic virus, or human T-cell leukemia-lymphoma virus (HTLV) family of viruses are a group of human retroviruses that are known to cause a type of cancer called adult T-cell leukemia/lymphoma and a demyelinating disease called HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). 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.