 Most proteins , ribosomal subunits , and some <unk> are transported through the pore complexes in a process mediated by a family of transport factors known as karyopherins . Those karyopherins that mediate movement into the nucleus are also called importins , whereas those that mediate movement out of the nucleus are called exportins . Most karyopherins interact directly with their cargo , although some use <unk> proteins . <unk> hormones such as <unk> and <unk> , as well as other small <unk> @-@ soluble molecules involved in <unk> signaling , can diffuse through the cell membrane and into the cytoplasm , where they bind nuclear receptor proteins that are <unk> into the nucleus . There they serve as transcription factors when bound to their ligand ; in the absence of ligand , many such receptors function as <unk> <unk> that <unk> gene expression . 
