CTSB and COVID-19: In line with these notions, the enrichment of all SARS-CoV-2 infection-related cellular modules (i.e., ACE2, TMRSS2 and CTSB, CTSL) in the gastrointestinal tract [17, 36] explain diarrhea as a major symptom of COVID-19 and SARS-CoV-2 RNA isolation from stool [33, 37, 38] (Fig. 2b).