The studies on the mechanism of HCV replication proliferated after the development of cell culture based-subgenomic HCV replicon system and genome-length HCV RNA replication system. Furthermore, these RNA replication systems have been improved to be suitable systems for the screening of anti-HCV reagents by the introduction of reporter genes such as luciferase. Genetic analysis of HCV RNAs obtained in long-term cell culture of HCV replicon or genome-length HCV RNA-harboring cells revealed that genetic mutations in HCV RNAs accumulated in a time-dependent manner. The genetic diversity of HCVs was also enlarged in a time-dependent manner. The appearance of adaptive mutation in HCV replicon or genome-length HCV RNA is one of characteristic features of HCV RNA replication system. Although human hepatoma-derived HuH-7 cell line was mainly used for HCV RNA replication systems, a specific combination of adaptive mutations led to develop the HCV RNA replication systems using a new human hepatoma cell line other than HuH-7.