IRF4^−/−Vh11 CLL cells are transplantable in immunodeficient host mice. CLL cells were isolated from spleens of IRF4^−/− Vh11 mice and transplanted into Rag2^−/−γ^−/− mice. A total of 10⁶ CLL cells was used for each injection. Three weeks after transplantation, the mice were analyzed. (A) Blood and spleen of the host mice were analyzed by FACS. (B) H&E staining of spleen of transplanted host mice. (C) CLL cells were isolated from spleens of host mice. TUNEL assay was used to detect apoptotic cells. Splenic B cells from IRF4^+/+Vh11 mice were analyzed as control. (D) Host mice were pulse-labeled with BrdU and examined 12 hours later. BrdU⁺ cells in blood and spleen of IRF4^+/+Vh11 mice were also examined and used as control. *P < .05; **P < .01.