Nuclear import and export of signaling molecules is an important step in the signal transduction pathways of cytokines and growth factors. Nuclear localization sequence(NLS) is the specific amino acid sequence on the signaling proteins to be sufficient for their nuclear translocation. In addition, nuclear pore complex(NPC),nuclear transport protein-importin and the energy-providing GTPase-Ran/TC4 are also involved in the nuclear import of the signaling molecules. NLSs are also widely located on the cytokines, growth factors or their receptors, and these ligands or receptors may act as chaperon molecules to assist nuclear translocation of other intracelluler signaling proteins through the action of NLS.