Rare development of B-cell lymphomas in CD19-CreER^T2× LoxP-Tag mice. (A) Kaplan-Meier survival graph of B-cell malignancies in CD19-Cre × LoxP-Tag and tamoxifen-treated CD19-CreER^T2 × LoxP-Tag mice. Tamoxifen was given at 6 to 8.5 weeks of age. Mice euthanized because of formation of non-B-cell tumor entities (n = 12) or other signs of distress (n = 4) were censored. CD19-CreER^T2 × LoxP-Tag mice were monitored until day 951, but no further B-cell tumor was detected. The log-rank test was performed. (B) Spleen and liver sections of tumor bearing CD19-Cre × LoxP-Tag (n = 12) and CD19-CreER^T2 × LoxP-Tag mice (n = 2) were stained for hematoxylin and eosin (H&E), PAX5, and Ki67. Shown are sections of 1 representative mouse. Bars represent 50 µm. (C) Splenocytes of tumor-bearing CD19-Cre × LoxP-Tag (n = 12) and CD19-CreER^T2 × LoxP-Tag mice (n = 3) or wt control (n = 1) were analyzed by flow cytometry for B-cell linage markers and TAg expression. FACS plots of representative mice are depicted.