B1 B cells were analyzed in the spleen and peritoneal cavity of CD19Cre Tg/+, Klf2+/+ mice (WT) or CD19Cre Tg/+, Klf2Fl/Fl (KO) mice that were (A) >10 weeks old (B) 8–11 days old or (C) 30 days old. The number or percentage of B1 B cells is indicated for each tissue. In (C), the number of B2 B cells is also indicated. These data are compiled from at least 3 experiments (the number of animals represented are 9 WT and 8 KO in A, 8 WT and 12 KO in B, 9 WT and 7KO in C).