Host cell DNAs or residual DNAs (rDNAs) are trace/low quantity of DNA originating from the organisms used in the production process of biopharmaceutical products, which may be introduced into the final products. The rDNAs may be able to transmit viral infections, cause a potential risk for oncogenesis or adverse reactions, etc. To assure the safety and potency, rDNAs in final biopharmaceutical products must be carefully monitored and quantitated, following the requirements established by World Health Organization (WHO), the European Pharmacopeia, the US Food and Drug Administration (FDA), and other regulatory agencies.