Because most of recombinant proteins are synthesized by cell-based systems in biochemical researches, the host cells derived from bacteria, yeast, mammalian cells, insect cells and plants (such as rice and tobacco) can be used for protein therapeutics manufacturing. But limited by current purification techniques, low levels (1 to 100 ppm) of host cell proteins (HCPs) may still remain in the purified biotherapeutics, even after a series of purifications. The ppm-level contaminants in biotherapeutics may trigger an unpredictable immune response in patients after dosing, and are required to be identified and quantified as part of drug safety evaluation, by the regulatory agencies.