CatL, a key human endosomal cysteine protease, cleaves the virus S1 subunit on spike glycoprotein at an appropriate acidic pH and facilitates the entry of SARS2-CoV2 into the host cell.15,320 Compared with healthy individuals, the circulating level of CatL is markedly higher in patients with SARS-CoV-2 infection and is associated with the status and severity of infection. Here, CTSL is linked to infection.