Protein transport across the nucleus is a selective, multi-step process involving several cytoplasmic factors. Proteins must be recognized as import substrates, dock at the nuclear pore complex and translocate across the nuclear envelope in an ATP-dependent fashion. Two cytosolic factors centrally involved in the recognition and docking process are the karyopherin a1 and karyopherin (beta)1 subunits. Karyopherin a1 functions in the recognition and targeting of substrates destined for nuclear import, while karyopherin (beta)1 serves as an adapter, tethering the karyopherin (alpha)1/substrate complex to docking proteins on the nuclear envelope termed nucleoporins. Karyopherin a2 has been shown to complex with Epstein-Barr virus nuclear antigen 1 (EBNA1). Certain RNA-binding proteins are imported to the nucleus by karyopherin (beta)2, and karyopherin (beta)3 appears to be involved in the import of some ribosomal proteins.