Mixed lineage leukemia (MLL)(KMT2A) rearrangements (KMT2A r) play a crucial role in leukemogenesis. Dependent on age, major differences exist regarding disease frequency, main fusion partners and prognosis. In infants, up to 80% of acute lymphoid leukemia (ALL) bear a MLL translocation and half of them are t (4; 11), resulting in a poor prognosis. In contrast, in adults only 10% of acute myeloid leukemia (AML) bear t (9; 11) with an intermediate prognosis. The reasons for these differences are poorly understood. Recently, we established an efficient CRISPR/Cas9-based KMT2A r model in hematopoietic stem and progenitor cells (HSPCs) derived from human cord blood (huCB) and faithfully mimicked the underlying biology of the disease. Here, we applied this model to HSPCs from adult bone marrow (huBM) to investigate the impact of the cell of origin and fusion partner on disease development. Both genome-edited infant and adult KMT2A r cells showed monoclonal outgrowth with an immature morphology, myelomonocytic phenotype and elevated KMT2A r target gene expression comparable to patient cells. Strikingly, all KMT2A r cells presented with indefinite growth potential except for MLL-AF4 huBM cells ceasing proliferation after 80 days. We uncovered FFAR2, an epigenetic tumor suppressor, as potentially responsible for the inability of MLL-AF4 to immortalize adult cells under myeloid conditions.