Molecular model of the cysteine protease rhodesain (ribbons) from the protozoan parasite Trypanosoma brucei rhodesiense bound to the inhibitor K777 (ball and stick model). T. brucei rhodesiense causes acute African sleeping sickness in humans. Rhodesain is involved in the parasite crossing the blood-brain barrier during infection and therefore K777 is being investigated as an antiparasitic drug. K777 has also been shown to block the SARS-CoV-2 virus, the cause of covid-19, from entering human cells and so is a potential treatment for the disease.