SARS-CoV-2 main protease, molecular model. The main protease of the SARS-CoV-2 virus, the cause of covid-19, an important role in processing the proteins needed to replicate the virus. The enzyme is made of two identical subunits; both are required for catalytic activity. As shown with stick structures, the active site includes catalytic dyad of cysteine and histidine side chains. The covid drug Nirmatrelvir (not shown) is an inhibitor of the main protease. Atoms are colour-coded: carbon is black, hydrogen is white, nitrogen is blue, and sulphur is yellow.