Definition

A mutated (changed) form of a type of gene called a proto-oncogene, which is involved in normal cell growth and division. When a proto-oncogene is changed so that too many copies are made or it becomes more active than normal, it is called an oncogene. Oncogenes may cause normal cells to become cancer cells and grow in the body. Mutations that lead to the conversion of proto-oncogenes to oncogenes usually occur during a person’s lifetime and are not inherited from a parent.