GCs occupied by AID^−/− B cells are more numerous and larger in immunized mixed BM chimeras. (A) Mixed BM chimeras were generated where irradiated hosts were reconstituted with a 1:1 mixture of WT (CD45.1) and AID^−/− (CD45.2) donor BM. Most GCs were occupied by both WT and AID^−/− B cells. However, some GCs were occupied by either WT or AID^−/− B cells exclusively as evidenced by staining for CD45.1 in the absence of CD45.2 in the GL7⁺ GC, or vice versa. GCs were enumerated (B) and measured for surface area (C) at day 7 and day 14 after immunization. Median values are shown in panel C. Eight mixed BM chimeric mice were evaluated at each time point. Statistics represent comparisons between WT GC versus AID^−/− GC. *P ≤ .05. ***P ≤ .001.