Thus, we first investigated if high levels of AID were present in in vivo preleukemic precursor B cells purified from mice carrying a genetic susceptibility to B-ALL (either Pax5 heterozygosity or the presence of the ETV6-RUNX1 fusion gene), which are exposed to natural infections (Supplementary Fig. 1a). This evidence concerns the gene RUNX1 and precursor B-cell acute lymphoblastic leukemia.