MUC4 and COVID-19: The sPLS-DA plot showed a clear discrimination among the different patient groups (Figure 4B), with MUC1 mRNA expression and age as major determinants for critically ill COVID-19 patients, whereas expression of MUC2, MUC20, and MUC21 mRNA were the best factors to identify patients with mild COVID-19 and sex, MUC4, MUC6, and MUC16 mRNA expression to identify non–COVID-19 patients with mild Illness (Figure 4C).