Pro-GC B cells that are intermediate between naive and GC B cells are predominantly RB⁺ and express higher levels of CD27. (A) Pro-GC B cells (IgD⁺CD38⁻ CD23⁻FSC^HI) were cytometrically identified from within the Q3 IgD⁺CD38⁻ quadrant and are considered to be developmentally intermediate between small resting naive B cells and early GC B cells.²⁰  Differential RB expression (%RB^−/lo and RB⁺) was determined for pro-GC B cells and small naive B cells that were either CD23⁻ or CD23⁺. (B) CD69 and CD27 MFI values were calculated for each fraction. The majority of small naive B cells were RB⁻ independent of CD23 expression. Pro-GC B cells were predominantly RB⁺ and expressed the highest level of CD27.