A gene that causes normal cell to become cancerous either because the gene is mutated or because the gene is expressed at the wrong time in development.
An oncogene is a gene that, when mutated or expressed at high levels, helps turn a normal cell into a tumor cell. Many abnormal cells normally undergo a programmed form of death (apoptosis). Activated oncogenes can cause those cells to survive and proliferate instead. Most oncogenes require an additional step, such as mutations in another gene, or environmental factors, such as viral infection, to cause cancer. Since the 1970s, dozens of oncogenes have been identified in human cancer. Many cancer drugs target those DNA sequences and their products.