SETDB1 preferentially silences evolutionarily young retroelements.

SETDB1 primarily silences evolutionarily young LTR retroelements through H3K9 methylation in mice.

• Why it matters: Understanding how transposable elements are epigenetically repressed is crucial because their activation can threaten genome stability. Current models lack detailed locus-specific insights into the mechanisms involved, especially for different TE subclasses.
• What they did: Using long-read RNA sequencing mapped to a complete mouse genome, the study examined the effects of SETDB1 and HUSH factor knockdowns on TE expression, chromatin modifications, and transcriptional activity at single-locus resolution.
• The result: Findings show SETDB1-dependent H3K9 methylation maintains silencing of young LTR retroelements, while LINE elements are repressed via broader heterochromatic pathways, revealing subclass-specific TE repression mechanisms.