A dominant mutant allele of a cellular gene (a proto-oncogene) that disrupts cell growth and division and is capable of transforming a normal cell into a cancerous cell. Proto-oncogenes typically encode proteins involved in positive control of the cell division cycle, such as growth factor receptors, signal transduction proteins, and transcription factors. Mutations in these genes tend to relax control mechanisms and accelerate cell division, leading to the cell proliferation that is characteristic of cancer. Some oncogenic mutations cause inhibition of programmed cell death (apoptosis), so that cancerous cells are less likely to be destroyed by the body’s defences. Most oncogenic mutations arise during the lifetime of the individual and affect body cells (i.e. somatic cell mutations). However, in rare cases germ-line cells are affected, so that offspring might potentially inherit the oncogene. Certain oncogenes of vertebrates are derived from viruses (see oncogenic). Compare tumour-suppressor gene.