Fas (APO-1, CD95) is a 45 kDa cell surface protein that mediates apoptosis when cross-linked with agonistic anti-Fas antibodies or Fas ligand (FasL). FasL is a tumor necrosis factor (TNF)-related type II membrane protein and is expressed in various tissue and cells. Both Fas and FasL are thoμght to play an important role in the apoptosis processes that takes place during T cell development.