Human promyelocytic leukemia cell line, HL-60, undergoes macrophagic differentiation when it is stimulated with TPA (12-O-tetradecanoylphorbol-13-acetate). We have cloned ETR101 cDNA whose mRNA was induced immediate early (30 min) and transiently by TPA. The mRNA is superinduced by addition of the protein synthesis inhibitor cycloheximide. The sequence of ETR101 cDNA (1826 base pairs) reveals that (i) it will encode a protein of 223 amino acids with a formula molecular weight of 24,200, (ii) the amino acid sequence is highly homologous to mouse chx1 protein whose mRNA was found recently to be enhanced in activated T lymphocytes in response to cycloheximide, (iii) the amino acid sequence is also weakly homologous to jun family gene products, and (iv) in the mRNA 3'-flanking region, there is a unique GUUUG sequence which is complementary to a part of B1 repetitive sequence and may be involved in mRNA degradation. ETR101 mRNA is induced by TPA in a wide variety of leukemia cells including myeloid, T-lymphoid, and B-lymphoid lineages. We have found that this mRNA is also induced by okadaic acid, a protein phosphatase inhibitor, and that TPA or cycloheximide act synergistically with okadaic acid. In addition, the induction is inhibited by protein kinase C inhibitors. Therefore, ETR101 mRNA level is controlled, either directly or indirectly, by protein phosphorylation.