MLLT3 and leukemia: Together with the cyclin-T1-CDK9 complex (P-TEFb), the most frequent MLL1 fusion partners AF4/AFF4, ENL/AF9 and ELL associate with each other on the AF4/AFF4 heterodimeric scaffold [10] and constitute SEC, which play essential roles in malignant gene expression, oncogenesis and maintenance of MLL1-r leukemia [15–17].