HXR9 hydrochloride is a cell-permeable peptide and a competitive antagonist of HOX/PBX interaction. HXR9 hydrochloride antagonizes the interaction between HOX and a second transcrip-tion factor (PBX), which binds to HOX proteins in paralogue groups1 to 8. HXR9 hydrochloride 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.