The first isolation and characterization of a human retrovirus was done by Gallo and co-workers from patients with aggressive cases of adult T cell leukemia/lymphoma in the United States [15, 16]. This virus, named human T cell leukemia/lymphoma virus (HTLV-I) is a T cell-tropic retrovirus [8, 9], initially found sporadically among United States cases of adult T cell leukemias and lymphomas (ATLL) with cutaneous manifestations resembling aggressive variants of mycosis fungoides. Subsequently, HTLV-I was specifically linked to adult T cell malignancies in Japan and the Caribbean [8]. A second human retrovirus, HTLV-II, which is related to but distinct from all previous HTLV-I isolates, was identified and isolated from a patient with hairy cell leukemia [12]. Recently, a third retrovirus, HTLV-III has been isolated and is a probable etiologic agent in the acquired immune deficiency Syndrome (AIDS) [4–7].