The oncoprotein 18 (Op18) gene encodes a proliferation-related cytosolic phosphoprotein, which is induced in normal lymphocytes following mitogenic stimulation. Studies of the Op18 gene are of particular interest because of the proposed role of Op18 protein in signal transduction and because of its occurrence in markedly increased amounts in acute leukemia cells. We have recently reported the cloning and sequencing of two cDNA clones for Op18 (1 and 1.5 kilobases). Both clones code for the same 149-amino acid polypeptide; however, they differ in their 3'-region as a result of alternative polyadenylation. We report here the sequencing of the Op18 gene and describe its expression in leukemia. The Op18 gene, which is 6.3 kilobases in length, is comprised of five exons and four introns and exhibits features that are common to other genes involved in cellular growth and proliferation. The increase in Op18 polypeptide in leukemia is associated with increased RNA transcription without gene amplification or rearrangement. Treatment of K562 leukemia cell line with hemin that induces terminal differentiation resulted in decreased expression of Op18. Our findings suggest that the high amount of Op18 protein in acute leukemia results from increased expression of a structurally unaltered gene.