Compared to HCs, patients with mild COVID-19 showed downregulation of a PPP-related gene (TKT), while patients in the severe COVID-19 group showed upregulation of glycolysis-related genes (LDHA, PGD, PGAM1, and PKM) and a fatty acid-related gene (HACD4) and downregulation of lysine degradation-related genes (NSD1, SETD2, KMT2C, KMT2E and KMT2A) in mono-CD14+ cells (Figure 2B). Here, HACD4 is linked to COVID-19.