Apoptosis regulator that functions through different apoptotic signaling pathways (PubMed:23429263, PubMed:26015568, PubMed:26949185, PubMed:27098698, PubMed:9535847). Plays a roles as pro-apoptotic protein that positively regulates intrinsic apoptotic process in a BAX- and BAK1-dependent manner or in a BAX- and BAK1-independent manner (PubMed:23429263, PubMed:26015568, PubMed:26949185). In response to endoplasmic reticulum stress promotes mitochondrial apoptosis through downstream BAX/BAK1 activation and positive regulation of PERK-mediated unfolded protein response (PubMed:26015568). Activates apoptosis independently of heterodimerization with survival-promoting BCL2 and BCL2L1 through induction of mitochondrial outer membrane permeabilization, in a BAX- and BAK1-independent manner, in response to inhibition of ERAD-proteasome degradation system, resulting in cytochrome c release (PubMed:9535847, PubMed:26949185). In response to DNA damage, mediates intrinsic apoptotic process in a TP53-dependent manner. Plays a role in granulosa cell apoptosis by CASP3 activation (By similarity). Plays a roles as anti-apoptotic protein during neuronal apoptotic process, by negatively regulating poly ADP-ribose polymerase-dependent cell death through regulation of neuronal calcium homeostasis and mitochondrial bioenergetics in response to NMDA excitation (PubMed:27098698). In addition to its role in apoptosis, may regulate trophoblast cell proliferation during the early stages of placental development, by acting on G1/S transition through regulation of CCNE1 expression.May also play a role as an inducer of autophagy by disrupting interaction between MCL1 and BECN1 (By similarity). Monomer; positively regulates apoptotic process. Homodimer (PubMed:23429263). Heterodimer (By similarity). Oligomer; promoted by apoptotic stimuli and BH3-only proteins; mediates constitutive activation (PubMed:26949185). Interacts (via BH4 domain) with ITPR1; enhances BOK expression and stabilization; limits apoptosis and prevents ubiquitination and then degradation; protects ITPR1 from proteolysis by CASP3 during apoptosis (PubMed:27053113, PubMed:23884412). Interacts with ITPR2 AND ITPR3; binds most strongly to ITPR2, and barely to ITPR3; regulates their expression (PubMed:23884412). Interacts with XPO1; translocates to the cytoplasm (By similarity). Interacts with BNIP3; promotes oligomerization (By similarity). Nuclear and cytoplasmic compartments in the early stages of apoptosis and during apoptosis associates with mitochondria. In healthy cells, associates loosely with the membrane in a hit-and-run mode. The insertion and accumulation on membranes is enhanced through the activity of death signals, resulting in the integration of the membrane-bound protein into the membrane (By similarity). The transmembrane domain controls subcellular localization; constitutes a tail-anchor (PubMed:23429263, PubMed:26949185). Localizes in early and late endosome upon blocking of apoptosis (PubMed:23429263). Must localize to the mitochondria to induce mitochondrial outer membrane permeabilization and apoptosis (PubMed:26949185). Widely expressed (PubMed:9535847, PubMed:23429263). Highly expressed in brain, kidney, and spleen (PubMed:27098698). At 15.0 dpc, expressed in brain, liver, thymus, lung, intestinal epithelium and follicles of the whiskers. Induced upon proteasome inhibition. BH4 domain mediates interaction with ITPR1. Ubiquitinated by AMFR/gp78 E3 ubiquitin ligase complex; mediates degradation by ubiquitin-proteasome pathway in a VCP/p97-dependent manner; prevents from proapoptotic activity; promotes degradation of newly synthesized proteins that are not ITPR1 associated. Knockout Bok mice have a normal development. 'Matador' means killer in Spanish. Belongs to the Bcl-2 family.