A cell-permeable peptidomimetic that prevents MLL1 (Mixed Lineage Leukemia 1) from complex assembling with WDR5 (Trp-Asp Repeat Domain 5), RbBP5 (Retinoblastoma Binding Protein 5), and ASH2L (Absent Small or Homeotic-2-Like) for enhanced H3K4 methyltransferase activity (IC50 = 0.4nM). Shown to reduce HoxA9 and Meis-1 mRNA expression in myeloblasts derived from MLL1-AF9-transduced murine bone marrow cells (50uM for 96 h) and preferentially inhibit the growth of MLL1-AF4-harboring MV4;11 and MLL1-ENL-harboring KOPN8 leukemia cultures (GI50 in 7 days = 25uM) via apoptosis induction, while exhibiting much reduced potency against BCR-ABL-dependent K562 cells (GI50 in 7 days = 84uM).