In contrast, perforin-deficient lupus-prone mice exhibit accelerated disease progression [13] and in graft-vs.-host murine lupus models both Fas and perforin are required for effective clearance of autoreactive B cells [14, 15], implicating CD8+ T cell cytolytic functions in maintenance of peripheral tolerance and halting autoimmunity. Here, FAS is linked to systemic lupus erythematosus.