In addition to a large body of data on molecular changes in tissues from COVID-19 patients, application of the above-mentioned techniques in autopsy tissues has also demonstrated protein expression that enable viral infection of the host cell, particularly the viral-entry receptor angiotensin-converting enzyme 2 (ACE2) and transmembrane protease, serine 2 (TMPRSS2), the cell-membrane-based protease needed for conformational changes in the SARS-CoV-2 spike protein leading to fusion with the host cell plasma membrane [152]. Here, ACE2 is linked to viral infectious disease.