The human Cdk2 complex is inactive as a protein kinase unless a. the cyclin subunit is degraded b. Malignant tumors generally require formation of new blood vessels.
Mammalian cells respond to addition of growth factors by turning on transcription of key genes.
All infected cells express unregulated and large amounts of the v-onc gene which leads to rapid loss of growth control in each of these cells. Since integration is more or less random in the complete genome, this is a relatively rare event and only a small fraction of initially infected cells lose growth control.