Telomerase, a ribonucleoprotein important to neoplastic immortality, is up-regulated in approximately 85% of cancers, including leukemias. In this study, 9cUAB30, a novel retinoic acid, resulted in differentiation of HL60 leukemia cells as indicated by morphologic changes characteristic of granulocytes. It also caused a down-regulation of hTERT gene expression and a decrease in telomerase activity. Telomerase inhibition was followed by loss of proliferative capacity, induction of apoptosis, and partial differentiation. These findings demonstrate the effectiveness of 9cUAB30 at inhibiting telomerase activity by down-regulating hTERT gene expression in human leukemic cells.