PBX3 and acute myeloid leukemia: Orally bioavailable, investigational or clinical drug candidate Menin inhibitors (MIs) disrupt binding of Menin to its binding pocket in MLL1/2 and MLL1-FP, which reduces MLL1/2 and MLL1-FP binding to their targets, inhibits HOXA9/MEIS1 activity, represses PBX3, MEF2C, FLT3 and CDK6, as well as induces differentiation and loss of survival of AML with MLL1-r or with mutant (mt)-NPM1 [2, 8–10].