The murine leukemia viruses (MLVs or MuLVs) are retroviruses named for their ability to cause cancer in murine (mouse) hosts. It is group/type VI retroviruse belonging to the gammaretroviral genus of the Retroviridae family. Some MLVs may infect other vertebrates. The MLVs include both exogenous and endogenous viruses. Exogenous forms are transmitted as new infections from one host to another. Endogenous MLVs are integrated into the host's germ line and are passed from one generation to the next. Scientists have classified murine leukemia viruses into four categories based on host specificity and determined by the genomic sequence of viral envelope region. These four categories include ecotropic MLVs, Non-ecotropic MLVs, polytropic MLVs and modified polytropic MLVs.