FAS and systemic lupus erythematosus: As has been shown previously in TACI deficient mice on C57BL/6 background41, TACI deficient lupus prone mice on MRL-Fas/Lpr background also had an elevated number of splenic B cells at 6 weeks of age as compared to wild-type MRL-Fas/Lpr mice (Supplemental Fig. 1A).