Proteins targeted to specific intracellular organelles such as mitochondria or the endoplasmic reticulum are able to cross membranes. Yet, to enter or exit the nucleus, proteins and RNA must pass through nonmembranous "gates" of the nuclear envelope, the nuclear pore complexes. Recently, the primary amino acid sequence of a few nuclear pore proteins (the nucleoporins) became available. Nucleoporins from mammals, amphibians and yeast are structurally homologous indicating that nuclear pore structures are evolutionarily conserved in the eukaryotic cell. The role of nucleoporins in nucleocytoplasmic transport is still unclear: are nucleoporins involved in decoding nuclear targeting signals or are they mere transporters? Although definite answers are not yet available, data are rapidly accumulating from several laboratories using a variety of approaches.