et al., 1980) and, together with cell proliferation, governs HenningWalczak1, Mariapia A.Degli-Esposti, the homeostasis of tissues (reviewed by Thompson, 1995). Richard S.Johnson, Pam J.Smolak, A subgroup of the TNFR family has been shown to be Jennifer Y.Waugh, Norman Boiani, critically involved in mediating apoptosis. The members Martin S.Timour, Mary J.Gerhart, of this subfamily, which includes TNFR-1, CD95 (APO-1/ Kenneth A.Schooley, Craig A.Smith, Fas), TRAMP (APO-3, DR3, WSL) (Chinnaiyan et al., Raymond G.Goodwin and Charles T.Rauch 1996; Kitson et al., 1996; Marsters et al., 1996b; Bodmer