The eleven-nineteen leukemia (ENL) protein family, composed of ENL and AF9, is a common component of 3 transcriptional modulators: AF4-ENL-P-TEFb complex (AEP), DOT1L-AF10-ENL complex (referred to as the DOT1L complex) and polycomb-repressive complex 1 (PRC1). Each complex associates with chromatin via distinct mechanisms, conferring different transcriptional properties including activation, maintenance, and repression. The mixed-lineage leukemia (MLL) gene often fuses with ENL and AF10 family genes in leukemia. However, the functional interrelationship among those 3 complexes in leukemic transformation remains largely elusive. Here, we have shown that MLL-ENL and MLL-AF10 constitutively activate transcription by aberrantly inducing both AEP-dependent transcriptional activation and DOT1L-dependent transcriptional maintenance, mostly in the absence of PRC1, to fully transform hematopoietic progenitors. These results reveal a cooperative transcriptional activation mechanism of AEP and DOT1L and suggest a molecular rationale for the simultaneous inhibition of the MLL fusion-AF4 complex and DOT1L for more effective treatment of MLL-rearranged leukemia.