FURIN and infection: CTSB is another important lysosomal cysteine protease involved in the infection of diverse viruses and host immune response.21,39 However, the results showed that 14a and 14b did not exhibit inhibitory activity against TMPRSS2 (at a concentration of 10 μM) and furin (at a concentration of 20 μM), two serine proteases involved in coronaviral entrance.