AFF4 and leukemia: Despite > 70 fusion partners of MLL1 were identified, only a few are frequently found in ~ 70% MLL1-r leukemias [1, 10–12], including transcription cofactors AF9 (also known as MLLT3) [13] and its paralog ENL (also known as MLLT1) [14], AF4 and its paralog AFF4 [15, 16], and ELL (Additional file 1: Figure S1a).