To investigate the role of DNA methylation in the regulation of gene expression, mRNA levels of Notch1-3, JAG1, Hes1, Hes2, Hes4 and Hes5 genes were analyzed by quantitative real-time-PCR in normal hematopoietic lineage cells, leukemia cell lines and patients primary bone marrow samples. The gene discussed is NOTCH1; the disease is leukemia.