Prdm1gfp/+ 11 (link), Prdm1fl 25 (link), Irf4fl 26 (link), Xbp1fl 22 (link) and Rosa26-CreERT2 27 (link) mice were bred and maintained at the animal facilities of the Walter and Eliza Hall Institute. Prdm1fl mice carried a loxP-flanked exon 5, while Prdm1gfp mice expressed an allele truncated after exon 5 (of a total of 7 exons). The PCR analysis for exon 5 loss has been described previously25 (link). All mice were on C57BL/6 background. Animal experiments were conducted according to the protocols approved by the Walter and Eliza Hall Institute ethics committee. Immunization was a single intraperitoneal injection of 100 μg NP-KLH precipitated onto alum (ThermoScientific). For cell transfer experiments, total splenocytes (5 × 106) or isolated B cells (2 × 106) were injected intravenously into Rag1−/− recipients. Cre-mediated deletion of loxP-flanked alleles was triggered by the administration of tamoxifen (Sigma-Aldrich, 0.2 mg/g) by oral gavage on two consecutive days.