As an extension of the observation that THL profoundly represses DNMT1 protein level in APL cells [5], we explored the effects of THL on MCF-7 human breast cancer cells possessing aberrantly elevated DNMT1 protein [7] in an attempt to elucidate its anticancer activities in association with DNMT1 downregulation. This evidence concerns the gene DNMT1 and acute promyelocytic leukemia.