A gene present in normal cells which when altered can transform the normal cell into a malignant cell. (When emphasising this relationship, the unaltered gene is called a proto-oncogene and the altered gene, an oncogene.) The alteration may result in overproduction of a gene product or faulty function. An oncogene is denoted by the prefix c if a cellular oncogene, e.g. c-ras, or by the prefix v if a viral oncogene, e.g. v-ras, which is presumed to have originated from a capture of the c-oncogene by the virus during an infection of a normal cell. Contrasted with a thanatogene, or death gene, which when activated leads to apoptosis.