During interphase, cyclin B1:Cdc2 shuttles continuously in and out of the nucleus. Cyclin B1:Cdc2 is transported into the nucleus by an unusual mechanism that requires importin b but not importin a or Ran. Dissociation of the cyclin-B1:Cdc2:importin complex in the nucleus requires ATP and involves
other yet unidentified nuclear factors (Takizawa et al.,1991).