Background High microvascular permeability plays an essential role in pathological process of multiple diseases such as septic shock,acute lung injury and acute respiratory distress syndrome,and burns.Inhibiting hyperpermeability is significant for controlling these conditions.Cdc42,as a main member of the small Rho GTPase family,plays a critical role in controlling and regulating the endothelial junctional permeability.We aimed to generate and identify endothelial specific cdc42-deficient mice by the Cre/Ioxp recombination approach,for examination in an animal model of the contribution of the cdc42 gene in the microvascular barrier function.Methods We crossed cdc42Flox/Flox mice with mice expressing endothelial cell-specific Cre recombinase,and the offspring with the genotype cdc42Flox/+Tie2Cre+/- were back-crossed with the cdc42Flox/Flox mice. The cdc42Flox/FloxTie2Cre+/- mice in the F2 generation were the target mice.If the cdc42 deficient mice did not survive,we would observe the cdc42 deficient mice embryos,and compare them with wild-type mice embryos.Results Cdc42Flox/+Cre+/- mice were mated with the cdc42Flox/Flox mice and among the living offspring there were no cdc42Flox/FloxCre+/- target mice.We found the endothelial special cdc42 deficient embryos at the E7.5-E16.5 stage.We observed that cdc42 deficient embryos were much smaller,had fewer vessels and were a little more swollen compared with the wild-type embryos.Conclusions Endothelial specific knockout of cdc42 caused embryonic lethality and the mice did not survive to birth.The target embryos were much smaller,had fewer vessels and were a little more swollen compared with the wild-type embryos.These results demonstrated that the cdc42 plays an important role in development of embryos and in development of microvessels as well as microvascular permeability.