The Human T-lymphotropic virus Type I (HTLV-1) is a human RNA retrovirus that causes T-cell leukemia and T-cell lymphoma in adults and may also be involved in certain demyelinating diseases, including tropical spastic paraparesis. The HTLV-1 genome is diploid, composed of two copies of a single-stranded RNA virus whose genome is copied into a double-stranded DNA form that integrates into the host cell genome, at which point the virus is referred to as a provirus. Alternative Names: HTLV 1, HTLV-1, HTLV-1 protein, HTLV 1 gp21 protein, HTLV1, HTLV 1 protein, HTLV-1 gp21 protein, gp21 protein, Human T Lymphotrophic Virus Type 1 glycoprotein 21 protein, Glycoprotein 21 protein