DPP4 and aortic valve calcification: Another proposed door for SARS-CoV-2 infection is the highly lung-expressed DiPeptidyl Peptidase-4 (DPP4) receptor, which has been found negatively regulated by melatonin in mice and in in vitro models of calcific aortic valve disease [105], suggesting the putative regulation of SARS-CoV-2 entry by the indolamine [106].