IRF4^−/−Vh11 CLL cells proliferate mainly in spleen and are resistant to apoptosis. (A) To characterize the proliferation of IRF4^−/−Vh11 CLL cells in vivo, we pulse-labeled the mice with BrdU. Twelve hours later, cells were isolated from blood, spleen, and lymph node and stained with antibodies against CD5, IgM, and BrdU. The BrdU-positive cells were revealed by FACS analysis. (B) Splenocytes were isolated from IRF4^−/−Vh11 and IRF4^+/+Vh11 mice and stained with CD5 and IgM. The apoptotic cells were detected by TUNEL assay. Values are averages and SDs of 3 independent experiments. *P < .01.