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. Typically, this signal consists of one or more short sequences of positively charged lysines or arginines exposed on the protein surface. Different nuclear localized proteins may share the same NLS. An NLS has the opposite function of a nuclear export signal (NES), which targets proteins out of the nucleus. Types Classical These types of NLSs can be further classified as either monopartite or bipartite. The major structural differences between the two are that the two basic amino acid clusters in bipartite NLSs are separated by a relatively short spacer sequence (hence bipartite - 2 parts), while monopartite NLSs are not. The first NLS to be discovered was the sequence PKKKRKV in the SV40 Large T-antigen (a monopartite NLS). The NLS of nucleoplasmin, KR[PAATKKAGQA]KKKK, is the prototype of the ubiquitous bipartite signal: two clusters of basic amino acids, separated by a spacer of about 10 amino acids. Both signals are recognized by importin α. Importin α contains a bipartite NLS itself, which is specifically recognized by importin β. The latter can be considered the actual import mediator. Chelsky et al. proposed the consensus sequence K-K/R-X-K/R for monopartite NLSs. A Chelsky sequence may, therefore, be part of the downstream basic cluster of a bipartite NLS. Makkah et al. carried out comparative mutagenesis on the nuclear localization signals of SV40 T-Antigen (monopartite), C-myc (monopartite), and nucleoplasmin (bipartite), and showed amino acid features common to all three. The role of neutral and acidic amino acids was shown for the first time in contributing to the efficiency of the NLS. Rotello et al. compared the nuclear localization efficiencies of eGFP fused NLSs of SV40 Large T-Antigen, nucleoplasmin (AVKRPAATKKAGQAKKKKLD), EGL-13 (MSRRRKANPTKLSENAKKLAKEVEN), c-Myc (PAAKRVKLD) and TUS-protein (KLKIKRPVK) through rapid intracellular protein delivery. They found significantly higher nuclear localization efficiency of c-Myc NLS compared to that of SV40 NLS. Non-classical There are many other types of NLS, such as the acidic M9 domain of hnRNP A1, the sequence KIPIK in yeast transcription repressor Matα2, and the complex signals of U snRNPs. Most of these NLSs appear to be recognized directly by specific receptors of the importin β family without the intervention of an importin α-like protein. A signal that appears to be specific for the massively produced and transported ribosomal proteins, seems to