FAS and autoimmune disease: Although loss of either Fas or Bim causes fatal autoimmune disease on certain genetic backgrounds (e.g., MRL for Faslpr/lpr [Watanabe-Fukunaga et al., 1992] or mixed C57BL/6x129SV for Bcl2l11−/− [Bouillet et al., 1999]), such pathology is not apparent for either mutation on an inbred C57BL/6 background.