The possible role of DNA methylation changes during several commitment steps of immature myeloid precursor cells toward functional, terminally differentiated phagocyte cells has previously been examined in the human myeloperoxidase (MPO) and macrophage colony-stimulating factor/c-fms genes using normal and transformed myeloid precursor cells. The human lysozyme (LZM) gene also provides a very useful model, because its protein synthesis is continuously increased during myelopoiesis and thus most abundant in mature phagocytes. Several shifts toward LZM gene demethylation coincide with upregulation of expression: activation of expression in myeloid precursor cells and in primary cells of acute myeloid leukemia (AML) was associated with demethylation at a CpG dinucleotide within the 5' flanking region; high-level expression in different types of normal mature phagocytic cells was associated with complete demethylation at two additional, intragenic CpG sites. Methylation changes occurring within the lysozyme gene could reflect transcriptional control of gene expression or maintenance of distinct maturation stages during phagocyte development. They correlate with maturational arrest and lysozyme gene expression in acute myeloid leukemias and may thus provide a genetic marker for the blocked differentiation of these neoplastic cells. Similar observations have been made for the MPO and c-fms genes.