Host cell proteins (HCPs) are process-related impurities, expressed by the host cell used for production of biopharmaceutical proteins. During the purification process, the majority of the HCPs are removed (>99%), but small HCP amounts remain in the distributed products, such as monoclonal antibodies (mAbs), antibody-drug-conjugates (ADCs), therapeutic proteins, vaccines, and other protein-based biopharmaceuticals.[1][2][3]