The therapeutic value of DNA-damaging antineoplastic agents is dependent upon their ability to induce tumor cell apoptosis while sparing most normal tissues. Here, we show that a component of the apoptotic response to these agents in several different types of tumor cells is the deamidation of two asparagines in the unstructured loop of Bcl-x_L, and we demonstrate that deamidation of these asparagines imports susceptibility to apoptosis by disrupting the ability of Bcl-x_L to block the proapoptotic activity of BH3 domain-only proteins. Conversely, Bcl-x_L deamidation is actively suppressed in fibroblasts, and suppression of deamidation is an essential component of their resistance to DNA damage-induced apoptosis. Our results suggest that the regulation of Bcl-x_L deamidation has a critical role in the tumor-specific activity of DNA-damaging antineoplastic agents.