Northern blot analysis of RNA isolated from HL-60 cells before and after differentiation induction by TPA and DMSO showed that four MPO mRNA species (3.3, 3.1, 2.7, and 2.5 kb, respectively designated alpha 1, beta 1, alpha 2, and beta 2) are expressed in HL-60 cells. However, alpha 2 and beta 2 lack part of the 3' end sequence due to different polyadenylation sites. The steady state levels of alpha 2 and beta 2 MPO mRNA increase significantly after 1 hr of induction, while all four MPO mRNA species decrease dramatically after 10 hr of induction. Our results demonstrate that MPO gene expression is developmentally and differentially regulated. Northern blot analysis of RNA isolated from blast samples of acute myelogenous leukemia (M0-M5) and chronic lymphocytic leukemia (CLL) patients indicate that four MPO mRNA species are expressed in M1-M4 but are undetectable in M5 and CLL. Primer extension and S1 nuclease protection analysis of the MPO mRNA revealed a single transcription initiation site for the MPO gene.