Relb^−/−cRel^−/− B-cell progenitors fail to respond to BAFF-mediated developmental signal in vitro. (A) Schematic of in vitro B-cell differentiation system. (B) Representative FACS plots of B220 and IgD expression in transitional B cells at the beginning (day 0) and end (day 4) of culture with or without BAFF. (C) Mean frequency of B220⁺IgD⁺ (T2-like) B cells following 4 days of culture as described in panel B. Numbers represent frequencies of live B220⁺ B cells in indicated gates. (D) Survival of in vitro cultured B cells. Live cells were identified by gating out 7AAD^Hi population. Fold survival calculated from initial B-cell population (day 0). For FACS plots, n = 4. wt, wild-type.