A subset of patients with acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) harbor rearrangements of the MLL gene, which are detected either by cytogenetic or fluorescent in situ hybridization evaluation at the time of diagnosis. A protein called DOT1L plays an important role in the malignant process in these leukemias. EPZ-5676 is a molecule that blocks the activity of DOT1L, and is therefore being evaluated in the treatment of patients with MLL-rearranged leukemias.