ACE2 and COVID-19: This hypothesis is supported by the fact that angiotensin-converting enzyme 2 (ACE2), and putatively also sialic acids, the suggested “doors” by which COVID-19 could enter endothelial cells and pericytes, are almost ubiquitarian, and not only present in the endothelial cells of alveolar membranes (Letko et al., 2020; Shang et al., 2020).