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Genetic diseases display a great deal of variability in patient outcomes, much of which is caused by differences in genetic background. The endoplasmic reticulum (ER) stress response commonly modifies degenerative disease...Endoplasmic reticulum (ER) stress-induced apoptosis is a primary cause and modifier of degeneration in a number of genetic disorders. Understanding how genetic variation influences the ER stress response and subsequent activation of apoptosis could improve individualized therapies and predictions of outcomes for patients. In this study, we find that the uncharacterized, membrane-bound metallopeptidase CG14516in Drosophila melanogaster, which we rename as SUPpressor of ER stress-induced DEATH(superdeath), plays a role in modifying ER stress-induced apoptosis. We demonstrate that loss of superdeathreduces apoptosis and degeneration in the Rh1G69Dmodel of ER stress through the JNK signaling cascade. This effect on apoptosis occurs without altering the activation of the unfolded protein response (IRE1 and PERK), suggesting that the beneficial prosurvival effects of this response are intact. Furthermore, we show that superdeathfunctions epistatically upstream of CDK5—a known JNK-activated proapoptotic factor in this model of ER stress. We demonstrate that superdeathis not only a modifier of this particular model, but affects the general tolerance to ER stress, including ER stress-induced apoptosis. Finally, we present evidence of Superdeath localization to the ER membrane. While similar in sequence to a number of human metallopeptidases found in the plasma membrane and ER membrane, its localization suggests that superdeathis orthologous to ERAP1/2in humans. Together, this study provides evidence that superdeathis a link between stress in the ER and activation of cytosolic apoptotic pathways.