Mechanism of tumorigenesis in chickens after infected with ALV-J. In host cells infected with the virus, the up-regulated circ-Vav3 increased the adsorption of gga-miR-375 through the miRNA sponge function, and further relieved the inhabitation of the gga-miR-375 target gene-YAP1. The increased accumulation of YAP1 can promote tumorigenesis through inducing EMT by affecting the mesenchymal markers-N-cadherin, Fibronectin, Vimentin, ZEB1, MMP2 and the epithelial marker-E-cadherin.