Aberrant DNA methylation patterns are commonly found in malignant cells. Hypermethylation of gene promotes sequences in cancer cells resulting in transcriptional silencing of tumor suppressor genes and contributing to malignant transformation. Hypomethylating agents have been explored for the treatment of cancers such as acute myelogenous leukemia and myelodysplastic syndrome. In vitro inhibition of methylation using azanucleosides results in differentiation of transformed myeloid cells. In vivo these agents induce DNA methylation of malignant myeloid cells. The most potent DNA methylating agent currently available, decitabine, is effective in patients with myelodysplastic syndrome (MDS) and acute myelogenous leukemia (AML). About 30% of patients with MDS and an abnormal karyotype will have normalization of their karyotype after receiving the drug.