Although biomanufacturing processes are designed to generate highly pure drug substances, some host-cell proteins (HCPs) copurify with target proteins and thus remain in finished drug products. Biopharmaceutical developers are keenly aware that such impurities must be minimized to protect patients. HCPs can activate several kinds of immune responses in treated patients, including production of antidrug antibodies and induction of cross-reactivity with therapeutic proteins (1–5). HCPs also can diminish drug efficacy, potency, and/or stability (6, 7). Thus, regulatory guidances such as…