HXR9 is a cell-permeable peptide and a competitive antagonist of HOX/PBX interaction. HXR9 antagonizes the interaction between HOX and a second transcrip-tion factor (PBX), which binds to HOX proteins in paralogue groups1 to 8. HXR9 selectively decreases cell proliferation and promotes apoptosis in cells with a high level of expression of the HOXA/PBX3 genes, such as MLL -rearranged leukemic cells.