Retroviral oncogenes are thought to form through errors in the replication of retrovirus genome following their integration at random sites into the host DNA end result is a viral gene which is structurally similar to its cellular counterpart is persistently different in its function. For example, the viral oncogene is almost to the gene for one chain of growth factor known as the platelet derived factor (pdgf). Cellular oncogenes are highly conserved in evolution.