This 2018 Michigan human cell study subject was factors affecting the expression of human endogenous retroviruses:

“We provide a comprehensive genomic and epigenomic map of the more than 500,000 endogenous retroviruses (ERVs) and fragments that populate the intergenic regions of the human genome.

The repressive epigenetic marks associated with the ERVs, particularly long terminal repeats (LTRs), show a remarkable switch in silencing mechanisms, depending on the evolutionary age of the LTRs:

• Young LTRs tend to be CpG-rich and are mainly suppressed by DNA methylation, whereas
• Intermediate age LTRs are associated predominantly with histone modifications, particularly histone H3 lysine 9 (H3K9) methylation.
• The evolutionarily old LTRs are more likely inactivated by the accumulation of loss-of-function genetic mutations.

Because the expression of ERVs is potentially dangerous to the host cell, understanding the repressive mechanisms is important. Earlier studies have implicated the aberrant expression of ERVs in autoimmune disease pathogenesis. However, this “enemy within” may also play a beneficial role in cancer therapy.

The same kinds of chromatin dynamics appear to be used both by LTRs and genes.”

I wasn’t going to curate this study before I saw the above graphic of our Boreoeutherian ancestor. Evolutionary subjects seem very abstract until an artist reconstructs the data visually.

https://genome.cshlp.org/content/early/2018/07/03/gr.234229.118.full.pdf “Switching roles for DNA and histone methylation depend on evolutionary ages of human endogenous retroviruses” (not freely available)

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