In MLL1-rearranged (MLL1-r) AML, N-terminus of the MLL1 gene is fused to the C-terminus of any of over 80 fusion partners, including AF4, AF9, ENL, and ELL, which are part of and recruit the super elongation complex (SEC) (including AFF1/4 and pTEFb) and DOT1L to induce H3K4Me3 and H3K79Me2 marks on active chromatin [2, 12–15]. Here, MLLT3 is linked to acute myeloid leukemia.