Apoptosis-inducing factor (AIF) was the first identified caspase-independent cell death effector. 1 However, despite a number of interesting studies implicating AIF in apoptosis (in vitro experiments with recombinant AIF, microinjection of neutralizing antibody, genetic invalidation, crystal structure, etc.) 2 its role as a caspase-independent effector remains unclear. Moreover, it was recently reported that the release of AIF occurs downstream of the release of cytochrome c in response to several proapoptotic stimuli, 3 increasing the controversy about AIF.

When Susin et al published the molecular characterization of AIF in 1999, they reported that overexpression of AIF induces programmed cell death (PCD). 1 When we overexpressed full-length AIF in HeLa, Cos-7 or 293T cell lines, we did not observe a significant apoptotic effect (Figure 1a) even 72 h after transfection (data not shown). We observed that AIF accumulates in the mitochondria and that the cells remain viable (Figure 1b), even in the presence of very high amounts of AIF. This suggests that mitochondrial AIF is not toxic for the cell and that only the cytosolic form of AIF is proapoptotic as previously reported. 2