Caspase-activation pathways. Additional pathways also exist, including (1) an apoptosis pathway induced by CTL and NK cells, in which serine protease Granzyme B is introduced into target cells; (2) an endoplasmic reticulum (ER) stress pathway, which involves caspase 12; and (3) a p53-induced pathway mediated by p53-inducible death domain (PIDD), which binds adaptor protein ICH-1/cell death protein-3 (CED-3) homologous protein with a death domain (RAIDD), an activator of caspase 2.