Human T-cell leukemia virus type 1 (HTLV-I) is etiologically associated with adult T-cell leukemia/lymphoma (ATL). The prototypic HTLV-I, ATK, is the only full-length provirus cloned from uncultured leukemic cells and completely sequenced prior to this study. We have determined the complete nucleotide sequence of another full-length HTLV-I provirus cloned directly from leukemic cells. A premature termination codon was found in the second open reading frame (orf II) of the pX region. Our finding indicates that open reading frame II of the HTLV-I pX region is not required for outgrowth of ATL leukemic clones in vivo.