By now, many of us are aware that a considerable portion (45% or more) of the human genome consists of transposable elements. These are mobile genetic sequences, such as Alu repeats and long and short interspersed nuclear elements (LINEs and SINEs). A whopping 18% of this so-called "dark matter of the genome" is retroviral sequences left over from ancient infections of germ line cells. This means that, in total, ~8% of the human genome is retroviral, compared to only ~1.5% that codes for human genes—a situation that led my Ph.D. advisor to claim that there are more retroviruses in us than there is us in us! Due to mutations, the viruses no longer function as viruses, but are a collection of broken parts.
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