In acute promyelocytic leukemia (APL), the oncogenic fusion protein PML/RARα causes the block of differentiation at promyeolocytic stage. Pharmacologic doses all-trans retinoic acid (ATRA) restore differentiation. The microRNA-mediated silencing process is shown to be important in the formation process of APL. In this work, ATRA leads to the repression of the PML/RARα-dependent miR-181a and miR-181b expression. In vitro studies showed the functional relevance of the microRNA cluster in APL. Mechanistic studies proved the direct binding of the two microRNAs to the 3´UTR of the tumorsuppressor RASSF1A. Finally, it could be shown that RASSF1A and the miR-181a/miR-181b cluster exert essential functions in APL and ATRA-induced differentiation by modulation of the cell cycle regulator Cyclin D1.