Before revision:  This provides an upstream mechanism for bistability in apoptosis, which is seen to be a consequence of biologically observed receptor trimerization. We analyze the bistabilitythresholds of the model, which furthermore possesses robustness of bistability , and provide a model assessment criterion using tools from algebraic geometry .
After revision:  This provides an upstream mechanism for bistability and robustness. At low receptor concentrations, the bistability is contingent on the trimerism of FasL. Moreover, irreversible bistability , and provide a model assessment criterion using tools from algebraic geometry .