In MLL1-r leukemia, chromosome translocation produces an oncogenic fusion protein consisting of the N-terminal DNA-interacting domains of MLL1 (residues 1- ~ 1400) fused with one of > 70 fusion partner proteins (Fig. 1a) [12–14], among which transcription cofactor proteins AF4 (∼36%), AF9 (∼19%) and its paralog ENL (∼13%), AF10 (∼8%), ELL (∼4%) and AF6 (∼4%) are the most frequent [14] (Fig. 1b). The gene discussed is MLLT3; the disease is leukemia.