The nucleic acid sequence of the 3' region of human T-cell leukemia virus type II (HTLV-II) proviral DNA was determined using a HTLV-II proviral clone that could be recovered as infectious, transforming virus. The sequence data indicate a region of unknown function of approximately equal to 1.6 kilobase pairs in the 3' region, analogous to the X region previously identified in human T-cell leukemia virus type I (HTLV-I). Three overlapping open reading frames are present in the X region of HTLV-II. One of these open reading frames, Xc, is most likely to encode a protein product, because it has greater predicted amino acid sequence homology (78%) with the X-IV region of HTLV-I and a greater percentage of its base differences with X-IV at the third nucleotide position of codons than do the other open reading frames. Sequences of the X-region that include the open reading frames are conserved in two deletion mutants of HTLV-II, which are associated with a subline of Mo cells with a decreased dependence on fetal bovine serum.