Oncogene is the gene in a tumor virus or in the cancerous cells which, when transferred into the other cells, can cause the transformation (note which only certain cells are susceptible to the transformation by any one oncogene). The functional oncogenes are not present in the  normal cells. The normal cell has several "proto-oncogenes" which serve certain normal functions, and which under the right circumstances can be activated to become oncogenes. The prefix "v-" indicates that the gene is obtained from a virus, and is usually an oncogene (like v-src , v-ras, v-myb , etc).