msDNA: the story of a unique satellite DNA molecule in bacteria.

msDNA, a multi-copy single-stranded DNA satellite, is required for retron-linked abortive defense against phage infection in bacteria.

• Why it matters: The biological role of msDNA had been unclear for decades, despite its widespread presence and use in genome engineering.
• What they did: Retrons produce msDNA via reverse transcription from an RNA template, and recent work links retron elements to phage protection using abortive infection.
• The result: The anti-phage property depends on msDNA, implying this satellite DNA is a functional component of retron-mediated antiviral defense.

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