RBP4 and COVID-19: The selected genes with the most negative regression coefficients, corresponding to lower expression in the Definite group, were FABP4, encoding a fatty acid-binding protein considered a marker of alveolar macrophages, whose expression in the lung decreases in patients with LRTI, including COVID-19 (30–32), and RBP4, encoding a retinol-binding protein, whose expression in the lung also sharply decreases following onset of LRTI (30) and whose expression by macrophages in vitro is depressed by inflammatory stimuli (33) (Figure 2C and Supplemental Table 2).