MUC21 and COVID-19: Single cell sequencing data of COVID-19 patients demonstrated that transmembrane mucins MUC1, MUC4, MUC13 and MUC21 are all highly upregulated in patients with active disease [30] and also in blood samples MUC1 and MUC2 mRNA expression was significantly elevated in critical and mild COVID-19 while MUC16, MUC20 and MUC21 were significantly downregulated in severe COVID [37].