One of the chief characteristics of the retrovirus life cycle is the appearance of provirus caused by integration of viral genome into the host cell genome, and its delivery stably to the next generation as a part of host germ line. This stable form is called endogenous retrovirus (ERV) and expressed by exogenous or endogenous factors. HERVs and MuERVs are present in humans and mice correspondingly, and their expressions frequently cause diseases. Several diseases such as cancer, autoimmunity and neurological disorders are related with HERVs. Therefore, various strategies should be established for the development of effective therapies for the suffering patients.