Nuclear protein sorting occurs through nuclear pore complexes or NPCs embedded in the nuclear membrane. Each NPC is a tight aqueous pore that allows the selective transport of folded peptides by dilating the channel diameter to accommodate large molecules as they squeeze through it. Nuclear protein sorting occurs in two steps — recognition and transport. During nuclear import, cytosolic receptors recognize and bind the cargo proteins via specific amino acid sequences called nuclear localization signals. Hydrophobic amino acid sequences of phenylalanine and glycine called FG repeats line the NPC channel to function as binding sites for nuclear transport receptors. The receptor-protein complex wriggles across the pore by repeated binding, dissociating, and rebinding to FG repeats. On the nuclear side, a small monomeric GTPase—Ran, bound to GTP, associates with the receptor-cargo complex. The receptor undergoes a conformational change to release the protein inside the nucleus and return to the cytosol for catalyzing the next round of protein transport. The export of proteins and RNA from the nucleus occurs by a similar process and involves nuclear receptors recognizing nuclear export signals on the cargo.