Nuclear export is a key step in the pre-ribosomal maturation pathway. Because of their large size and their high production rate (around 4000 subunits/min), pre-ribosomal particles represent a major substrate for NPC crossing. Using high-pressure freezing and electron tomography, we were able to observe RNP particles translocating through the nuclear pore complex (NPC) in yeast S. cerevisiae. To our knowledge, this is the first example of RNP translocation through the NPC observed in yeast by transmission electron microscopy. Through collaboration with Pr. J-Y Dauxois, at INSA Toulouse, we have developed a statistical model of the dynamics pre-ribosome export .