Endogenous retroviruses (ERVs) have established themselves in the human genome in the course of evolution and play a major role in normal gene regulation. Excessive ERV activity, however, can lead to diseases such as autoimmunity and cancer. And so, cells have developed mechanisms for recognizing and silencing endogenous retroviruses. The silencing is accomplished by packaging the corresponding DNA sections into a less accessible structure. This process is facilitated by modifications to the histone proteins that package the DNA (H3K9me3) and a modification to the DNA itself (DNA methylation). It has not been clear before now, however, which of these modifications are really important for ERV silencing.
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