Importins and exportins are transport receptors carrying cargo molecules into and out of the nucleus. Exportin 4 is a member of the Importin B superfamily of RanGTP-binding proteins. Its homology to other family members is restricted to its N-terminal RanGTP-binding motif. Exportin 4 is a nuclear export receptor for eIF-5A, a protein involved in the cellular stress response, functioning as a regulator of p53-mediated and TNF-alpha-mediated apoptosis.