Researchers have tagged different SARS-CoV2 virus strains with their own unique fluorescent proteins e.g. the delta strain expresses GFP, while omicron variant expresses DsRed. The fact that cells infected by the virus will fluoresce, allows scientists to monitor COVID-19 infections. It also allows them to determine how effective potential anti-COVID drugs are – no fluorescence means no infection, which is an indication that the drug is effective.