HOXA9 and leukemia: Together with the cyclin-T1/CDK9 complex (also known as P-TEFb), these proteins associate with each other and constitute super elongation complexes (SEC) [15–17], which promote malignant gene expression (e.g., HoxA9, Meis1 and Myc) in MLL1-r leukemia and play critical roles in the cancer initiation and maintenance (Additional file 1: Figure S1b).