TMPRSS2 and COVID-19: Finally, considering the very recent interest in studying natural compounds and raw extracts as anti-COVID-19 agents [27,28,29], in the present study we exposed the human H1299 lung adenocarcinoma cell line to the P. mahaleb extract in order to measure the gene expression of angiotensin-converting enzyme 2 (ACE2) and transmembrane protease serine 2 (TMPRSS2), which are known to play a master role in mediating SARS-CoV-2 virus entry in the human host [30,31].