A strain of primate t-lymphotropic virus 1 isolated from mature T4 cells in patients with T-lymphoproliferation malignancies. It causes adult T-cell leukemia (leukemia-lymphoma, t-cell, ACUTE, HTLV-I-ASSOCIATED), T-cell lymphoma (lymphoma, t-cell), and is involved in mycosis fungoides, sezary syndrome and tropical spastic paraparesis (paraparesis, tropical spastic).