MI-503 is a potent and selective inhibitor of the menin-MLL (mixed lineage leukemia) interaction, making it a valuable tool in cancer research, particularly in leukemias driven by MLL gene rearrangements. It disrupts the binding between menin and MLL fusion proteins, which is critical for the oncogenic gene expression program in MLL-rearranged leukemia cells. This inhibition leads to suppression of key target genes such as HOXA9 and MEIS1, resulting in reduced proliferation and induction of apoptosis in cancer cells. Due to its specificity and cellular activity, MI-503 is widely used in preclinical studies to validate the menin-MLL interface as a therapeutic target. It has shown promising antileukemic effects in cell line models and animal studies, supporting the development of menin inhibitors as a potential treatment strategy for acute leukemias with MLL translocations. Additionally, MI-503 serves as a pharmacological probe to study epigenetic regulation and protein-protein interactions in transcriptional control.