Alkylating DNA-damage agents such as N-methyl-N 0 -nitro-N 0 -nitrosoguanidine (MNNG) trigger necroptosis, a newly defined form of programmed cell death (PCD) managed by receptor interacting protein kinases. This caspase-independent mode of cell death involves the sequential activation of poly(ADP-ribose) polymerase-1 (PARP-1), calpains, BAX and AIF, which redistributes from mitochondria to the nucleus to promote chromatinolysis. We have previously demonstrated that the BAX-mediated mitochondrial release of AIF is a critical step in MNNG-mediated necroptosis. However, the mechanism regulating BAX activation in this PCD is poorly understood. Employing mouse embryonic knockout cells, we reveal that BID controls BAX activation in AIF-mediated necroptosis. Indeed, BID is a link between calpains and BAX in this mode of cell death. Therefore, even if PARP-1 and calpains are activated after MNNG treatment, BID genetic ablation abolishes both BAX activation and necroptosis. These PCD defects are reversed by reintroducing the BID-wt cDNA into the BID Ă/Ă cells. We also demonstrate that, after MNNG treatment, BID is directly processed into tBID by calpains. In this way, calpain non-cleavable BID proteins (BID-G70A or BID-D68-71) are unable to promote BAX activation and necroptosis. Once processed, tBID localizes in the mitochondria of MNNG-treated cells, where it can facilitate BAX activation and PCD. Altogether, our data reveal that, as in caspase-dependent apoptosis, BH3-only proteins are key regulators of caspase-independent necroptosis. Cell Death and Differentiation (2012) 19, 245-256; doi:10.1038/cdd.2011.91; published online 8 July 2011When and how a cell dies is one of the essential questions to understand the biology of the cell. A cell is considered dead when the integrity of its plasma membrane is compromised, when its fragments are engulfed by an adjacent or a professional cell or when its components, including the nucleus, are fragmented. It is more complicated to outline how a cell could die. Historically, two major types of cell death have been distinguished: apoptosis, a controlled or programmed cell death (PCD), and necrosis, an uncontrolled or accidental death. 1 Cell biologists have particularly focused their attention on apoptotic PCD, helping to characterize it at both genetic and biochemical levels. 2 Fascinatingly, the use of apoptotic inhibitors or cells that are deficient in key apoptotic molecules has revealed the existence of new PCD pathways. As a consequence, active forms of cell death started to be referred to as caspase-dependent or caspase-independent. 3 More recently, it has been accepted that necrosis is more than an unregulated type of death. Indeed, a cell can use different mechanisms/pathways with underlying apoptotic or necrotic features to accomplish its proper demise. 4 The analysis of the tumor necrosis factor (TNF) signaling path has shed new light on the existence of PCD pathways with necrotic features. When TNF binds its receptor, two ways can be engaged: (i) caspase-dependent a...