LAD2/RBD supernatant promotes the expression of MMP9 in alveolar epithelial cells, which resembles the observation of significantly increased circulating MMP9 level in COVID-19 patients.63,64 In acute lung injury, the released MMP9 are able to promote degradation of the alveolar-capillary barrier, through probably action on tight junction proteins.65 The gene discussed is MMP9; the disease is COVID-19.