In virus-producing cells, the proteolytic processing SARS-CoV-2 spike protein at the S1/S2 (two subunits of spike protein) boundary/cleavage site (for furin) required a higher expression of MMP9 and MMP2 proteases, which is associated with hyperinflammation and lung tissue damage in patients with COVID-19 patients [44]. Here, MMP9 is linked to COVID-19.