Compound deletion of RelB and cRel results in developmental block at the T1 stage. (A-C) FACS analysis of peripheral B-cell development in whole splenic extracts from wild-type, Relb^−/−, cRel^−/−, and Relb^−/−cRel^−/− mice. Identification of mature (B220⁺AA4.1⁻) and transitional B cells (B220⁺AA4.1⁺). Mature B cells (B220⁺AA4.1⁻) are further classified into FO B cells (CD21⁺IgM⁺) and mature MZ B cells (CD21^highIgM⁺). Transitional B cells (B220⁺AA4.1⁺) are subdivided into T1 B cells (CD23⁻IgM⁺) and T2 B cells (CD23⁺IgM⁺). Scatter plots are graphical representation of FACS plots: wild-type (●), Relb^−/− (▪), cRel^−/− (▲), and Relb^−/−cRel^−/− (▼). (D) Histologic analysis of splenic sections taken from wild-type and Relb^−/−cRel^−/− mouse using hematoxylin and eosin stain (H&E). (E) In vivo analysis of mature B-cell development in double knockout mouse using immunofluorescence of frozen splenic sections stained with anti-CD3 fluorescein isothiocyanate, anti-B220 allophycocyanin, and anti-MOMA-1 Alexa Fluor 405. *P < .05; **P < .005; ***P < .001. Also see supplemental Figure 2.