MLLT1 and acute lymphoblastic leukemia: Precursor-B ALL cells are blocked at early stages of B-cell differentiation due to the presence of various genetic lesions that give rise to different fusion oncoproteins such as E2A-PBX1 (TCF3-PBX1), BCR-ABL1, MLL-ENL (KMT2A-MLLT1), TEL-AML1 (ETV6-RUNX1) or others.