Importins and exportins
	• After RNA molecules (mRNA, tRNA and rRNA) are produced in the nucleus, they must be exported to the cytoplasm for protein synthesis.  In addition, proteins operating in the nucleus must be imported from the cytoplasm.  The traffic through the nuclear envelope is mediated by a protein family which can be divided into exportins and importins.  Binding of a molecule (a "cargo") to exportins facilitates its export to the cytoplasm.  Importins facilitate import into the nucleus.