Cytochrome c binds to the WD-40 domain of Apaf-1. This allows for the "lock" to be released, meaning this domain is no longer autoinhibited. However, the CARD and NB-ARC domains remain in autoinhibited state. The CARD domain will only be released from this lock when Apaf-1 is bound to (d) ATP/ATP; when ATP binds, the CARD domain will then be allowed to bind to Caspase-9. When ADP is in the ATPase domain, oligomerization is inhibited. Thus, the binding of ATP also allows for the oligomerization of Apaf-1 into the heptagonal structure necessary for downstream caspase activation. Mutations in the ATPase domain render the protein inactive; however, the method of controlling this ADP-ATP exchange is unclear.