W e examined the kenetics of p15 methylation and expression during myeloid development. We treated human cord blood CD34⁺ cells with either GM-CSF alone or in combination with stem cell factor and followed methylation at this locus using bisulfite genomic sequencing. CD34⁺ cells were found to be either fully methylated or completely unmethylated at 27 CpG dinucleotide sites in exon 1 and at 18 CpG sites in the promoter region of the p15 gene. A time-course study showed that the percentage of the allelic methylation of p15 CpG island increased to approximately 50% to 60% until 7 days after cytokine stimulation, then decreased to less than 10% after 21 days. The methylation was also observed in bone marrow CD34⁺ cells exposed to GM-CSF. p15 expression varied inversely with methylation. Expression was negligible or at low levels until 14 days, after which it increased substantially. The frequency of myeloid colony-forming cells in the progeny decreased and myeloid-specific markers increased in the later stages. Based on our observations on cells grown with GM-CSF and 5-aza-2′-deoxycytidine, DNA methylation of the p15 promoter region CpG island appears to be associated with proliferation rather than differentiation of normal human myeloid progenitors.