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An importin is a protein involved in selective import through a nuclear pore complex.

It recognizes suitable cargo directly or through adaptors and participates in passage through the selective pore environment. Directionality depends on the Ran nucleotide cycle and compartment-specific interactions rather than on a permanently one-way opening. ^[ref]

A nuclear localization sequence is an ordered stretch of amino acid residues that can direct protein import into a cell nucleus. In the classical pathway, importin α recognizes the nuclear localization sequence, and importin β mediates passage through the nuclear pore complex. Other recognition pathways exist. An exposed nuclear localization sequence is a targeting feature, not a guarantee of import under every condition.^[ref]

RNA export is a different cargo pathway: RNA travels associated with proteins, as a ribonucleoprotein, rather than being imported by its own nuclear localization sequence. Export of messenger RNA connects nuclear transcription to cytoplasmic biological translation.^[ref]