– Bid, BH3-interacting domain death agonist, was initially cloned based in its ability to interact with both Bcl-2 and Bax. Bid contains only the BH3 domain, which is required for its interaction with the Bcl-2 family proteins and for its pro-death activity. Bid is susceptible to proteolytic cleavage by caspases, calpains, Granzyme B and cathepsins. Bid is important to cell death mediated by these proteases and thus is the sentinel to protease-mediated death signals.