However, the exposure to natural infection does not significantly increase AID expression in preleukemic precursor B cells (Supplementary Fig. 1a), although in vitro exposure of preleukemic Pax5-het, and Sca1-ETV6-RUNX1 precursor pro-B cells to different immune activation stimuli resulted in high levels of AID mRNA (Supplementary Fig. 1b). Here, ETV6 is linked to infection.