Precursor B-cell differentiation patterns of healthy donors, NBS, XLA, and RAG-deficient SCID patients. (A) Flow cytometric analysis of the precursor B-cell compartment in bone marrow of 31 healthy donors and 18 NBS patients. (B) The average precursor B-cell differentiation pattern of the 18 NBS patients was compared with the average patterns of 14 XLA patients and 17 RAG-deficient SCID patients and the 31 controls. All 3 patterns showed a block before the pre-B-II cell stage; however, only in RAG-deficient SCID patients was this block complete. In NBS and XLA patients, the block was incomplete (“leaky”), but the percentage of immature B cells was much lower in XLA than in NBS.