iPCs express Blimp-1 and Bcl-6 mutually exclusively and the minority expressing both are maintained by p18 and Bcl-xL. (A) qPCR analysis of relative mRNA levels in sorted p18^+/+ and p18^−/− populations. (B) Single-cell qPCR analysis of Bcl-6 and Blimp-1 expression in aBCs, iPCs, and PCs from wild-type, p18^−/−, and Bcl-xL^Tg/+ mice on day 7 after NP-CGG immunization. Percentages were calculated from the total number of cells expressing Bcl-6, Blimp-1, or both.