PAX5 and leukemia: We next attempted to model the TCF3-HLF translocation defining a highly aggressive leukemia subtype for which so far no representative mouse model could be engineered.15 Given the frequent PAX5 heterozygous deletions and reduced gene expression identified in this leukemia,16 we tested the hypothesis if the Tcf3-Hlf translocation could induce a mouse leukemia when established in a Pax5 haploinsufficient context (Supplementary Fig. 3A–C).