are very abundant in primate genomes, they easily serve as phylogenetic markers for linkage analysis. They can be exploited by comparing the integration site polymorphisms or the evolving, proviral, nucleotide sequences of orthologs. To estimate when integration occurred, researchers used distances from each phylogenetic tree to find the rate of molecular evolution at each particular locus. It is also useful that ERVs are rich in many species genomes (i.e. plants, insects, mollusks, fish, rodents, domestic pets, and livestock) because its application can be used to answer a variety of phylogenetic questions. Designating the age of provirus and the time points of species separation events This is accomplished by comparing the different HERV from different evolutionary periods. For example, this study was done for different hominoids, which ranged from humans to apes and to monkeys. This is difficult to do with PERV because of the large diversity present. Further research Epigenetic variability Researchers could analyze individual epigenomes and transcriptomes to study the reactivation of dormant transposable elements through epigenetic release and their potential associations with human disease and exploring the specifics of gene regulatory networks. Immunological problems of xenotransplantation Little is known about an effective way to overcoming hyperacute rejection (HAR), which follows the activation of complement initiated by xenoreactive antibodies recognizing galactosyl-alpha1-3galatosyl (alpha-Gal) antigens on the donor epithelium. Risk factors of HERVs in gene therapy Because retroviruses are able to recombine with each other and with other endogenous DNA sequences, it would be beneficial for gene therapy to explore the potential risks HERVs can cause, if any. Also, this ability of HERVs to recombine can be manipulated for site-directed integration by including HERV sequences in retroviral vectors. HERV gene expression Researchers believe that RNA and proteins encoded for by HERV genes should continue to be explored for putative function in cell physiology and in pathological conditions. This would make sense to examine in order to more deeply define the biological significance of the proteins synthesized. See also Avian sarcoma leukosis virus (ASLV) Endogenous viral element Endogeny (biology) ERV3 HERV-FRD Horizontal gene transfer Jaagsiekte sheep retrovirus (JSRV) Koala retrovirus (KoRV) Mouse mammary tumor virus (MMTV) Murine leukemia virus (MLV) and xenotropic murine leukemia virus-related virus (XMRV) Paleovirology References Further reading External links HERVd – human endogenous retrovirus database Molecular biology Swine diseases Non-coding DNA