We found that a number of proteins involved in TJ formation and cell–cell adhesion junctions were drastically down-regulated in COVID-19 patients, including TJ protein ZO-1, TJ protein ZO-2, claudin-2, claudin-3, claudin-11, claudin-19, Afadin, cingulin, protein crumbs homolog 3, cAMP-dependent protein kinase catalytic subunit alpha (PRKACA), and Rho GTPase-activating protein 17 were drastically down-regulated in COVID-19 patients (Fig. 2b, Supplementary Fig. 10, and Supplementary Data 7). Here, CLDN2 is linked to COVID-19.