Host cell proteins (HCPs) are impurities present in all therapeutics derived from biological sources, and as a critical quality attribute (CQA), they must be characterized in detail and controlled through the manufacturing process and in the final drug product. The individual protein contaminants that make up the HCP profile may vary significantly among individual biologic products including cell and gene therapies and expression systems, even for very closely related molecules. As such, the HCP profile must be analyzed to identify even low levels of potentially concerning species in a biologic drug.