Human colour perception changes between seasons

have also played a role in the evolution of host genomic regulatory complexity. At each end of the ERV genome are long terminal repeats (LTRs), which contain regulatory sequences that can alter the expression, splicing, and polyadenylation of those host genes located near the ERV insertion site. LTRs regulate the cell type that the virus replicates in by controlling its expression, and so can be co-opted by their hosts as alternative promoters, resulting in tissue-specifi c expression of host genes. Often, solitary LTRs have been generated by homologous recombination between the two LTRs present in a single ERV, resulting in loss of the internal sequence. Consequently, host genomes are peppered with solo LTRs of potential regulatory signifi cance. Intriguingly, the LTRs of an ERV in primates (HERV-H) can bind pluripotency transcription factors that lead to the expression of the retrovirus, which in turn regulates stem cell identity. Taken together, the evidence suggests that sequences sequestered from ERVs have had a considerable infl uence on the evolution of their vertebrate hosts. So, not only is evolution a tinkerer, but it is also a conscientious recycler.