 Finally, in order to return to the cytoplasm, importin-α must associate with a Ran-GTP/CAS (nuclear export factor) complex which facilitates its exit from the nucleus.CAS (cellular apoptosis susceptibility protein) is part of the importin-β superfamily of karyopherins and is defined as a nuclear export factor.Importin-β returns to the cytoplasm, still bound to Ran-GTP.