QUOTE : The litter of ERVs across vertebrate genomes, mainly in the form of defective and deleted proviruses, mutationally inactivated proteins coding sequences, and isolated LTRs, confirms that the vast majority of endogenized retroviral genones decay to nonfunctional sequences. If they have been silenced by cellular regulatory mechanisms or if their gene products are not under purifying or positive selective pressures, their DNA sequences will be under no selective pressure to retain their functionality. Over time, they accumulate random mutations and their sequences drift without consequence to the host. That ERVs mostly become such inconsequential DNA is a topic of hot debate; it is, however, an easy matter to pick several examples that illustrate how hosts can benefit from their ERVs. As for all matters of evolution, we bear witness only to the successful events that are now fixed in genomes. Evolutionary failures, no matter they far outnumber the successes, go unrecorded, rapidly purged from the gene pool.