Examination of BM B cell populations from Hem1–/– and littermate control mice demonstrated severe B cell lymphopenia starting at the pre-pro–B cell developmental stage (Hardy fraction A) and extending through the mature recirculating FO B cell (B220hiCD43–IgM+; Hardy fraction F) stage (Figure 1, A and B). The gene discussed is NCKAP1L; the disease is lymphopenia.