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In page Nuclear pore complex:

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Nuclear proteins are synthesized in the cytoplasm and need to be imported through the NPCs into the nucleus. Import can be directed by various signals, of which nuclear localization signal (NLS) are best characterized.[1] Several NLS sequences are known, generally containing a conserved sequence with basic residues such as PKKKRKV. Any material with an NLS will be taken up by importins to the nucleus.[citation needed]