The present invention provides double-stranded RNA molecules that mediate RNAinterference in target cells, preferably hepatic cells. The invention alsoprovides double-stranded RNA molecules that are modified to be resistant tonuclease degradation, which inactivates a virus, and more specifically,hepatitis C virus (HCV). The invention also provides a method of using thesemodified RNA molecules to inactivate virus in mammalian cells and method ofmaking modified small interfering RNAs (siRNAs) using human Dicer.