The nuclear envelope (NE) separates the nuclear compartment from the cytosol and possesses unique sieving capabilities: substances which are destined to reside in the nucleus are specifically transported there while everything else remains excluded. This presents a potential problem for nuclear delivery of therapeutic molecules: if a drug does not reach its target inside the nucleus, it is rendered essentially ineffective. The substance exchange across the NE is mediated by nuclear pore complexes (NPCs). Despite the fact that extensive structural and biochemical data on the NPC is currently available, data concerning its barrier function is controversial. Itwas shown that over a half of the total mass of the domains which are believed to be responsible for the barrier function can be deleted without loss of the NPC's normal permeability barrier. It is our goal to resolve this controversy. Using the amphiphilic alcohol cyclohexanediol we artificially induced a breakdown of the permeability barrier. The diffusion of a fluorescent tracer and intranuclear proteins across the NE confirmed this breakdown. Structural details of a dilation of the NPC channel were imaged using atomic force microscopy. Our data suggests that the breakdown of the permeability barrier is due to an unexpected dissociation of a subset of NPC proteins. We already identified the protein Nup62 and we expect to determine the identity of the other dissociating proteins. This will provide crucial insight into understanding the barrier function of the NPC.