When the RNA genome of SARS-CoV-2 is translated into proteins for the first time, just after viral entry, a long protein is produced, called a poly-peptide, which needs to be cleaved into functional parts.
Without this, the virus cannot replicate and the infection cannot take hold. This is done by two proteases, nsp3 and main protease (also known as 3C-like protease or nsp5). Both therefore are major drug targets.
Main protease works as a dimer made up of two identical proteins, a homo-dimer, the picture shows these two copies (green) with a potential inhibitor bound (reddish).