Fetal liver and adult mouse HSPCs exhibit different dependencies on Id1 for MLL-AF9–mediated leukemogenesis. Retroviral transduction of fetal liver HSPCs and adult bone marrow with MLL-AF9 induces acute myeloid leukemia (AML); however, deletion of Id1 in fetal liver HSPCs significantly attenuates leukemogenesis, whereas deletion of Id1 in adult bone marrow HSPCs promotes leukemic development. Despite these differences, in both cell contexts, p21, a target of Id1, mediates these effects.