The intrinsic apoptotic pathway is activated by intracellular death signals, such as oxidative stress and DNA damage. In mammals, the pathway is regulated by the Bcl-2 family of proteins, which has both pro- and anti-apoptotic family members. Proapoptotic proteins include BH3-only proteins, such as Bad and Bim, and effector Bcl-2 family proteins, such as Bax and Bak. Anti-apoptotic proteins include Bcl-XL and Bcl-2 and inhibit Bax and Bak activity. Death signals activate the BH3-only proteins, which can either inactivate anti-apoptotic proteins or directly bind to effector proteins. In either case, these effectors oligomerize on the mitochondrial membrane and form pores that release cytochrome c. Cytochrome c binds to apoptotic protease activating factor-1, Apaf-1, and Apaf-1 monomers then assemble into a heptameric complex, called the apoptosome. Next, the initiator procaspase-9 binds to the apoptosome and dimerizes, forming caspase-9. Finally, caspase-9 activates the executioner caspases that cleave cellular proteins, resulting in apoptosis.