The key role of TMPRSS2 in COVID-19 is corroborated by the observation of an increased TMPRSS2 expression in the bronchial epithelial cells of male patients compared with female patients [72], which could provide an underlying explanation for the previously reported finding of an independent association of male sex with severe COVID-19 [73,74,75]. This evidence concerns the gene TMPRSS2 and COVID-19.