Protein that must be imported to the nucleus from the cytoplasm carry nuclear localization signals (NLS) that are bound by importins. An NLS is a sequence of amino acids that acts as a tag. They are diverse in their composition and most commonly hydrophilic, although hydrophobic sequences have also been documented. Proteins, transfer RNA, and assembled ribosomal subunits are exported from the nucleus due to association with exportins, which bind signaling sequences called nuclear export signals (NES). The ability of both importins and exportins to transport their cargo is regulated by the small Ras related GTPase, Ran.