CCR5 and HIV-1 infection: Since the replication of CCR5-tropic vif-deficient HIV-1 infection in BLT mouse needs the restoration of vif and is achieved only via artificial infection route (i.e., direct injection into the human thymic organoid, which is a unique organ in BLT mouse), it would be infeasible for CCR5-tropic HIV-1 to propagate in vivo via relatively natural infection routes (e.g., intrarectal, intravaginal, or intravenous infections).