The human T-cell leukemia viruses HTLV-I and HTLV-II are unique among the transforming retroviruses of vertebrates in their ability to transform human T cells in vitro and in their close association with human malignancies (T-cell lymphomas and leukemia). Their genomes are relatively simple, containing the genes gag, pol, env, and a 3' region termed "X." This 3' region may be responsible for the transforming potential of the viruses. The existence of proteins encoded by the 3' region has been postulated on the basis of multiple open reading frames. In the present study this region is shown to contain a gene encoding a protein of 40 kilodaltons in HTLV-I and 37 kilodaltons in HTLV-II. It is proposed that these proteins be called, respectively, p40xI and p37xII.