In the absence of cytochrome c, Apaf-1 exists in its monomeric form; it is thought that the WD-40 domain remain folded back onto the protein, keeping Apaf-1 in an auto inhibited state. In addition, several areas are so tightly bound that the protein is unable to bind to anything else. It has been determined through mass spectrometry that in the autoinhibited, or "locked" state, ADP is bound to the ATPase domain of Apaf-1. In this state, this protein is singular, and incapable of activating any caspases.