Proteins encoded by the fas and fas ligand (fasL) genes are involved i
n apoptotic cell death in lymphocytes. In this article we review the r
ecent elucidation of the role of the Fas-FasL interactions in the main
tenance of tolerance to self antigens and in the homeostatic regulatio
n of lymphocyte clonal expansion, and discuss the mechanisms of autoim
munity in Fas- and FasL-deficient mutant mouse strains.