AICDA and infection: Our previous study demonstrated that not all of the IgM memory cells expressed mutated BCRs, so it is also possible that not all of the precursor cells transcribed Aicda. To identify Aicda-positive cells early during early infection, infected (AID-cre-ERT2x EYFP) F1 mice were administered tamoxifen on days 4 and 7 post-infection, and splenic B cells were analyzed for EYFP expression three days later, on day 10 post-infection.