The X-ray crustal structure (PDB ID 6LU7) of the main protease (M Pro ) of SARS_CoV_2, the protein is displayed as cartoon diagram showing the domain I (in red color), domain II (in green color), domain III (in yellow color), and loop (in magenta color). The active site cavity remains at the cleft between domain I and II, the catalytic dyad (Cys145 and His41) are displayed in orange color, the other crucial residues namely Met45, Tyr54, Phe140, His163, Met165, Asp166, Phe185, and Glu189 are shown as sticks. The subsites (S1, S1', S2, S4) are shown by arrow marks.