Northern blot analysis of four typical cases of acute promyelocytic leukemia showed that one of the cell population examined was characterized by a very high level of expression of the myeloperoxidase (MPO) gene. Western blot analysis confirms that the protein content of the cells corresponded to the levels of the MPO mRNA. Southern blot studies of the DNA of this cell population ruled out the presence of any genome amplification or rearrangement. Chromosome hybridization studies in situ confirmed that the MPO gene was translocated on the long arm of chromosome 15. The observation that a typical genomic pattern may or may not be associated with the MPO overexpression leads us to believe that so far it is impossible to reach any conclusion about the significance of the translocation in the genesis of MPO overexpression.