Characterization of rare Blimp1^gfp/gfp tumors. (A) Representative histologic appearance of tumors by Blimp1 genotype. (Top) PCT in a Blimp1^gfp/+ mouse, characteristic of tumors seen in all Blimp1^+/+ and Blimp1^gfp/+ mice. (Middle) Reticulum cell sarcoma, as seen in 4 Blimp1^gfp/gfp mice. (Bottom) Lymphoid leukemia in Blimp1^gfp/gfp mouse no. 28. Images were viewed using a Zeiss Axiophot microscope (Oberkochen, Germany) with a Plan-apo 20×/0.60 objective and hematoxylin and eosin stain and captured with a Zeiss Axiophot camera. (B) Flow cytometric analysis of BM, spleens, and enlarged LN (“tumors”) of 2 representative Blimp1^gfp/gfp mice that developed tumors. The tissues of mouse no. 21 had a pattern characteristic of mice that bore reticulum cell sarcomas (nos. 17, 21, and 22). Mouse no. 28 had a lymphoid leukemia. The panel on the right for mouse no. 28 is gated on GFP⁺ cells in the tumor. (C) Quantitative reverse transcription PCR measuring expression levels of genes in normal resting B cells, GFP⁻ B-cell blasts, GFP⁺ plasmablasts generated in vitro (²⁷ ) and GFP⁺ cells sorted from 4 Blimp1^gfp/+ and all 5 Blimp1^gfp/gfp tumors. Note that the Blimp1 PCR amplifies exons that are not affected by the Blimp1/GFP modification, so both Blimp1⁺ and Blimp1^gfp alleles are detected.