SARS-CoV-2 Nucleocapsid protein (NP) is one of the core components of SARS-CoV-2 virus. It forms a complex with viral genomic RNA in a helical symmetrical structure and plays a key role in the process of virus replication and assembly. Since NP is abundantly expressed during infection, it can be used as an important diagnostic marker for COVID-19 and also can be used as a potential drug target or developing vaccines. Terbium-labeled anti-Tag5 antibody serves as fluorescence donor, that binds to the Tag5-NP. If a test mouse antibody binds to NP, fluorescence- labeled anti-mouse antibody (fluorescence acceptor) will be brought in close proximity with the fluorescence donor. Excitation of Terbium (340 nm) generates fluorescence resonance energy transfer (FRET) to the fluorescence-labeled acceptor, which consequently fluoresces at 665 nm (figure below). Thus, the test antibody binding to NP can be quantitively measured by calculation of the fluorescent ratio of 665 nm/620 nm.