Human T-cell lymphotropic virus, illustration. Human T-cell lymphotropic virus (HTLV) is a family of human RNA (ribonucleic acid) retroviruses that is known to cause a type of cancer called adult T-cell leukaemia/lymphoma (ATL) and a demyelinating disease called HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). The HTLVs belong to a larger group of primate T-lymphotropic viruses (PTLVs). The envelope glycoproteins on the surface of the virions are used by the virus to adhere to cells for invasion.