Nuclear pores and nuclear assembly

between the nucleus and cytoplasm occurs through large macromolecular struc tures, the nuclear pores. Quantitative scanning transmission electron micro scopy has estimated the mass of a nuclear pore to be 60 million Daltons in yeast and 120 million Daltons in vertebrates. The past two years were notew orthy in that they saw: 1) the purification of both the yeast and vertebrat e nuclear pores, 2) the initial description of routes through the pore for specific transport receptors, 3) glimpses of intranuclear organization impo sed by the nuclear pores and envelope and 4) the revelation of new and pivo tal roles for the small GTPase Ran not only in nuclear import but in spindl e assembly and nuclear membrane fusion.