IL2RG mutant naive B cells can express AICDA and differentiate into Ig-secreting cells in response to diverse stimuli, but not IL-21. Naive B cells from control donors (A; n = 5; B-E; n = 14) or transplanted X-SCID/JAK3–deficient patients with either donor-derived (group 1; A, n = 2; B-E, n = 6) or autologous (group 2; A, n = 3; B-E, n = 7) B cells were stimulated with CD40L alone or together with IL-4, IL-10, CpG, or IL-21. (A) Expression of AICDA was determined by quantitative PCR after 5 days. The levels of secreted IgM (B-D), IgG (E), and IgA (F) were determined by ELISA after 12 days. Each symbol represents AICDA expression of Ig secretion by naive B cells from an individual normal donor or patient (*P < .05).