The nuclear pore complex can be considered to be the stationary phase
of bidirectional traffic between the nucleus and the cytoplasm. The mo
bile phase consists of karyopherins, transport substrates, and the sma
ll GTPase Ran and its modulators. Recently, the family of karyopherins
was expanded with the recognition of numerous open reading frames wit
h limited homology to karyopherin beta 1. In several cases, the specif
ic substrates transported by the new karyopherins have been identified
, allowing the characterization of new pathways into and out of the nu
cleus. However, the mechanisms of transport, particularly the role of
Ran, remain poorly understood.