AFF1 and acute leukemia: Under physiological conditions, AFF1, MLLT1, and ELL associate with pTEFb (Positive Transcription Elongation Factor b) to form the Super Elongation Complex (SEC), which releases RNA Polymerase II (Pol II) from pausing to facilitate transcriptional elongation.16,17 In the context of acute leukemia, KMT2A-AFF1 and KMT2A-MLLT1 fusion proteins inappropriately recruit other components of the SEC to MLL target genes to promote and sustain the transcription of leukemogenic self-renewal genes such as HOXA9.