HACD4 and COVID-19: The CD16+ monocytes in the mild COVID-19 group expressed higher levels of glycolysis-related genes (PGAM1 and GAPDH), a fatty acid-related gene (HACD4), and an arginine and proline metabolism-related gene (SAT2), as well as lower levels of a cysteine and methionine metabolism-related gene (AHCYL1) (Figure 2B).