RNA interference (RNAi) was first reported in nematodes in 1998. Since that time, RNAi has been discovered in fish, insects and mammals. Novel treatments and drug discovery in preclinical studies based on RNAi are targeting a wide range of diseases at present, including viral infections and cancers. In addition to the local administration of synthetic small interfering RNA (siRNA) targeting local lesions, specific methods for the systemic administration of these molecules to treat infectious diseases or metastatic cancers have also been reported. In vivo delivery technology is a key hurdle that must be addressed for the successful clinical application of synthetic siRNA. In this review, the authors evaluate the recent findings on atelocollagen-siRNA complexes for the treatment for metastatic cancers and outline the cancer therapies and drug discovery studies that are based on RNAi technology.