To functionally demonstrate the role of IL-6, we impaired the expression of IL-6 by breeding Pax5+/- mice to IL-6+/− mice, which have significantly lower IL-6 serum concentrations (Fig. 1a), with the aim of testing whether IL-6+/-/Pax5+/- mice are more resistant to infection-induced B-ALL than Pax5+/- mice. Here, IL6 is linked to precursor B-cell acute lymphoblastic leukemia.