RanGAP1 is a Ran GTPase-activating protein that plays a pivotal role in the majority of nucleocytoplasmic transport pathways.The protein is limited to somatic cells.In tlis study,the localization and possible functions of RanGAP1 were examined during mouse oocyte fertilization.Immunofluorescence analysis showed that after sperm penetration,RanGAP1 was found to diffuse within the cytoplasm,but concentrated in the microtuble of the reversed spindle and the constriction ring between the oocyte and the second polar body;with the expansion of sperm chromatin,RanGAPl began to move to the region around the expanding sperm and oocyte chromatin,and gradually concentrated around the growing parents pronuclei.After the male and female pronuclei apposed,the membrane of one pronuclei broke first,numerous concentrated RanGAP1 dots were observed in the chromosome region.With the chromatin condensing into chromosome,the parents chromosomes mixed together and prepared to start the first mitosis,the condensed RanGAP1 was just the shape of the microtuble to assemble the first mitosis spindle.These showed that RanGAP1 played an important role in regulating spindle functions,chromosome alignment,PB2 extrusion and pronuclei nuclear envelope assembly/disassembly in mouse oocyte fertilization.