HXR9 hydrochloride, a cell-permeable peptide and competitive antagonist of the HOX/PBX interaction, selectively reduces cell proliferation and induces apoptosis in cells with elevated HOXA/PBX3 gene expression, notably MLL-rearranged leukemic cells [1] [2] [3]. By inhibiting the interaction between HOX proteins and the transcription factor PBX, which associates with HOX proteins in paralogue groups 1 to 8, it disrupts a critical regulatory mechanism in these cells.