Diffusion of metabolites and small non-nuclear molecules as well as active, mediated import of protein and export of protein and RNA through the nuclear envelope occurs through nuclear pore complexes or NPC's. NPC's contain up to 100 different polypeptides which have a combined mass of about 125 megadaltons. The channel available for passive transport through the NPC is about 9-10 nm in diameter while carrier mediated changes in the NPC result in a ~25 nm channel used for larger, actively transported molecules. Of the 100 polypeptides, at least 8 of these are O-linked N-acetylglycosamine-modified in mammalian cells. All of the mammalian O-linked glycoproteins contain multiple copies of phenylalanine, glycine dipeptide repeats dispersed throughout part of their sequence. Studies indicate that the NPC O-linked glycoproteins have a direct role in nuclear protein import.