The coordination of cell functions requires that molecules move in the cell interior to find their partners and eventually to form complexes.  In the cell interior, the mechanisms for molecular motion are poorly understood.  While in an isotropic fluid diffusion is the default mechanism of motion, in the cell interior diffusion is hindered by barriers and by transient binding.  Also molecules can move by active transport.  One universal transport processes which is still debated is the shuttling of molecules between the cytoplasm and the nucleus.  It is well established that molecules must pass through the nuclear pore complexes to go from the cytoplasm to the nucleus.  The structure of the nuclear pore complex has been elucidated.  The nuclear pore has the additional capability to selectively allow specific molecules to be transported between the cytoplasm and the nucleus. Despite many years of research, the physical mechanism responsible for the nuclear transport and pore selectivity remains elusive.