To address the underlying mechanisms by which theMef−/−Rad50s/s mice develop plasma cell neoplasms, weanalyzed the CSR capability ofMef−/−Rad50s/s B cells, using flow cytometric analysis ofIgG1+ B cells after a 4 day ex vivo stimulation withanti-CD40 Ab (1 μg/mL) and IL-4 (100 ng/mL).Mef−/−Rad50s/s B cells showed more IgG1+cells poststimulation than did the control,Mef−/−, orRad50s/s B cells, demonstrating their enhancedCSR capacity (Fig. 4a). Here, CD40 is linked to plasma cell neoplasm.