In recent years apoptosis, also called programmed cell death, has been recognized to be the physiological way for a nucleated animal cell to die. Apoptosis takes care of unwanted, injured or virus-infected cells (Farber 1994; Collins 1995). Autoreactive T and B cells that are produced by the immune system by the millions every day are also eliminated by apoptosis. A large number of stimuli can trigger apoptosis. However, only the discovery of the existence of receptors that could trigger apoptosis convinced everyone that a certain substance would not just kill a cell due to its high toxicity but involve special apoptosis-inducing mechanisms. A number of receptors that were first shown to have other functions besides induction of apoptosis could kill cells. These receptors include the T cell receptor/CD3 complex (Smith et al. 1989; Takahashi et al. 1989; Newell et al. 1990), the B cell receptor (Ales-Martinez et al. 1992), CD2 (Merkenschlager and Fisher 1991), CD4 (Wadsworth et al. 1990) and the mouse antigen Thy-1 (Ucker et al. 1989).