Due to the known impaired maturation of Irf4−/− preB cells [7], spontaneous eruption of preB-leukemia seemed plausible: In spleen sections (Fig. 1e), infiltrating cells stained positive for both B220 and Igμ (although less than untransformed “follicle” B cells) and Ki67. The gene discussed is CD40LG; the disease is leukemia.