Gammaretrovirus is a genus in the Retroviridae family. Example species are the murine leukemia virus and the feline leukemia virus. They cause various sarcomas, leukemias and immune deficiencies in mammals, reptiles and birds. Introduction Many endogenous retroviruses, closely related to exogenous gammaretroviruses, are present in the DNA of mammals (including humans), birds, reptiles and amphibians. Many also share a conserved RNA structural element called a core encapsidation signal. The avian reticuloendotheliosis viruses are not strictly avian viruses—it appears that reticuloendotheliosis viruses are mammalian viruses that were accidentally introduced into birds in the 1930s during research on malaria. As a potential vector for gene therapy, gammaretroviruses have some advantages over HIV as a lentiviral vector. Specifically, the gammaretroviral packaging system does not require the incorporation of any sequences overlapping with coding sequences of gag, pol, or accessory genes. Gammaretroviruses have a wide range of implications for animals. They have been linked with several diseases including cancer, specifically leukemias and lymphomas, various neurological diseases, and some immunodeficiencies in many different species. Gammaretroviruses are similar to other retroviruses and reverse transcribe a positive single strand RNA into double stranded DNA. The double stranded DNA is highly stable and easily integrated into a host genome. A few examples of the virus are Moloney murine leukemia virus, xenotropic MuLB-related virus, feline leukemia virus, and feline sarcoma virus. Gammaretroviruses are very popular retroviral vectors in laboratory studies. These vectors are crucial for gene therapy and gene transfer. The reason that they are so useful is because their genomes are very simple and easy to use. Retroviruses have the ability to integrate into host cell genomes very well, which allows for the long term expression of their genome. One specific gammaretrovirus that is commonly used as a retroviral vector is the Moloney murine leukemia virus. A specific gammaretrovirus called xenotropic murine leukemia virus-related virus (XMRV) has been found to infect prostate cancer tissue in laboratories. XMRV is a recombinant virus created in a laboratory accident in the mid-1990s. Although it can infect human tissue, no known disease is associated with the infection and it is unlikely to exist outside laboratories. Alleged discovery of XMRV in blood cells of patients with chronic fatigue syndrome in 2009 caused a controversy and eventual retraction. There were over 50 human cancer cell lines that were claimed to be linked to murine leukemia virus-related virus or murine leukemia virus. There have also