Micro RNA (miRNA) is a single-stand RNA composed of 21-23 nucleotides, which may regulate the function of massager RNA (mRNA). The regulating mechanisms involving micro RNA between the hosts and the HCV virus include (1) auto-regulation of HCV mRNA by HCV miRNA; (2) regulation of host mRNA by HCV miRNA; and (3) regulation of HCV mRNA by host miRNA. MiR-122 is the abundant liver-specific miRNA which is crucial for efficient HCV replication in culture Huh7 cells stably expressing HCV replicons. This observation raised much interest in the role of mir-122 in HCV infection and its potential as a therapeutic target. It was recently reported that the levels of miR-122 and several other miRNAs are regulated by IFN in Huh7 cells and primary mouse hepatocytes and that miRNAs might mediate at least some effects of IFN on HCV RNA replication in vitro. Recently, an in vivo study for hepatic miR-122 of 42 patients with CHC who received IFN-based therapy showed that patients who did not respond to IFN therapy had markedly decreased pretreatment miR-122 levels. Although miR-122 is abundant in the liver, liver biopsy is still considered an invasive procedure, which prevents its widespread use in routine clinical practice. Considering the abundant blood flow through the liver, we speculated that miR-122 can be detected in the sera, and can be used for screening and monitoring the responses by serial blood tests. The miRNA can be detected in the sera and is stable after 24 hours of room temperature store or repeated freezing and de-freezing. In addition, miRNA is resistant to RNase and can be sustained for more than 10 years after adequate freezing. Recent studies have shown the serum miRNA was potentially useful for the detection of cancer and its treatment responses. Furthermore, the serum miR-122 levels can reflect the severity of liver injuries in a rat acetaminophen toxicity model. Because miR-122 is liver specific and the miRNA is stable in the sera, the investigators aimed to evaluate the role of serum and hepatic miR-122 on the viral kinetics and the treatment responses and in HCV patients receiving peginterferon and ribavirin combination therapy.