Here, we report the biochemical reconstitution of a transcriptional elongation system using purified human proteins, AID, Pol II and DSIF, to investigate mechanisms of AID scanning and catalysis during transcription of IGHV3-23*01, which is the most commonly used variable region during normal immune responses (36,37) and in chronic lymphocytic leukemia (38). This evidence concerns the gene AICDA and B-cell chronic lymphocytic leukemia.