Mature naive B cells from IPEX patients show an increased proliferative history. (A) Representative CD80, CD86, and CD69 expression on CD19⁺CD27⁻ naive B cells in IPEX patients (solid gray) compared with healthy donors (bold). (B) Mean fluorescence intensities (MFI) of CD80, CD86, and CD69 expression on naive B cells from healthy donors (n = 7) and IPEX patients (n = 7). (C) Evaluation of the number of cell divisions undergone in vivo by KREC analysis on new emigrant and mature naive B cells of healthy donors (n = 41) and IPEX patients (n = 6).