Ribonuclease protection assay showing siRNA expression specifically degraded intracellular HCV positive strand RNA in the transfected Huh-7 cells. Huh-7 cells were co-transfected with different concentration of siRNA74 with different full-length clones of HCV. After 48 hours, transfected cells were isolated by the treatment with trypsin-EDTA. Total RNA was isolated and subjected to RPA for positive strand HCV using a minus strand RNA probe targeted to the 5'UTR region. The degradation of HCV positive strand by siRNA74 is concentration dependent. No HCV RNA degradation was observed in the cells transfected with unrelated siRNA.