The underscores were the sites for restriction digest.

下划线是限制性酶切位点。

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Note: Underline sequences are cleavage sites of restriction enzyme.

注：标有下划线的碱基序列为限制酶切位点。

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Restriction enzyme: Protein (more specifically, an endonuclease) produced by bacteria that cleaves DNA at specific sites along its length.

限制性内切：由细菌产生的一种蛋白质，能在特定的地方切断去氧核糖核酸分子。收藏。

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He also points out that YouTube was the first site to implement 10-minute limits on the lengths of clips--a restriction that has prevented the full-feature piracy common on other sites.

FORBES: YouTube's Filter Fails to Please