MUC4 and COVID-19: MUC1, MUC2, MUC4, MUC16, and MUC20 mRNA expression strongly correlated with COVID-19 severity, of which MUC1 and MUC20 mRNA expression also associated significantly with age (Figure 6, A–C) and MUC16 and MUC20 mRNA expression with sex (Figure 6A).