© 2016 American Association for Cancer Research.Homeobox (HOX) proteins and the receptor tyrosine kinase FLT3 are frequently highly expressed and mutated in acute myeloid leukemia (AML). Aberrant HOX expression is found in nearly all AMLs that harbor a mutation in the Nucleophosmin (NPM1) gene, and FLT3 is concomitantly mutated in approximately 60% of these cases. Little is known about how mutant NPM1 (NPM1mut) cells maintain aberrant gene expression. Here, we demonstrate that the histone modifiers MLL1 and DOT1L control HOX and FLT3 expression and differentiation in NPM1mut AML. Using a CRISPR/Cas9 genome editing domain screen, we show NPM1mut AML to be exceptionally dependent on the menin binding site in MLL1. Pharmacologic small-molecule inhibition of the menin–MLL1 protein interaction had profound antileukemic activity in human and murine models of NPM1mut AML. Combined pharmacologic inhibition of menin–MLL1 and DOT1L resulted in dramatic suppression of HOX and FLT3 expression, induction of differentiation, and superior activity against NPM1mut leukemia. SIGNIFICANCE: MLL1 and DOT1L are chromatin regulators that control HOX, MEIS1, and FLT3 expression and are therapeutic targets in NPM1mut AML. Combinatorial small-molecule inhibition has synergistic on-target activity and constitutes a novel therapeutic concept for this common AML subtype.