PDCD1 and hepatitis C virus infection: In this respect, the SMASh degron system is suitable because it employs a drug such as ASV or GRV, which is already used in the treatment of human hepatitis C. In order to investigate the effectiveness of degron-tagged PD-1 in humans, it is necessary to conduct experiments using immune-deficient mice transplanted with human HSCs carrying the PD-1-based degron system.