BAFF-deficient mice display a greater than 90% loss of B cells in all stages beyond the T1 stage,23 which can be compensated by the overexpression of anti-apoptotic proteins such as members of the Bcl-2 family.24 Comparable B-cell lymphopenia develops when mice are deficient for BAFF-R, which is normally expressed by all mature B cells.25 These observations prove that the BAFF/BAFF-R axis plays a vital role in the maturation and survival of B cells. The gene discussed is TNFSF13B; the disease is lymphopenia.