A nuclear localization signal or sequence (NLS) is an amino acid sequence that 'tags' a protein for import into the cell nucleus by nuclear transport.[1] Typically, this signal consists of one or more short sequences of positively charged lysines or arginines exposed on the protein surface.[1] Different nuclear localized proteins may share the same NLS.[1] An NLS has the opposite function of a nuclear export signal (NES), which targets proteins out of the nucleus.