An oncogene is a kind of abnormal gene that predisposes cells to develop into cancers. Unlike normal genes, which can be turned off after being turned on, oncogenes are altered in a way that keeps them stuck in a state of constant activity. That uninterrupted action helps drive the uncontrolled growth that underlies tumors.

Oncogenes can be turned on by inherited changes - ones that are passed down from parent to child - or by cancer-promoting agents that damage DNA throughout an individual's lifetime, such as UV light, hazardous chemicals and even viruses.

a diagram of an oncogene and its role in cancer
Cancer arises in part through damage to normal genes (known as “proto-oncogenes”), which can arise from exposures to cancer-promoting agents, such as UV light. That damage permanently switches the gene on, transforming it into an oncogene that helps push cells to become cancerous.
Image courtesy of Sigrid Hart, Broad Institute

The first oncogene ever identified is called src (pronounced "sarc"). Discovered in 1970, it is a component of a cancer-causing virus in chickens, known as the Rous sarcoma virus, which induces tumors in connective tissues, such as bone and muscle, in infected animals. In humans, the SRC gene was later shown to be involved in a variety of cancers, such as colon, liver, lung, breast and pancreatic cancer.

The src discovery ushered in a series of other landmark findings on oncogenes and how they operate in cancer, including the research of Harold Varmus and J. Michael Bishop. In 1975, these scientists showed that the oncogenes carried by viruses, like src and others, are not actually viral genes. Instead, the genes come from normal cells and are co-opted by viruses. This finding, as well as others, transformed the way scientists think about genes and cancer: they understood that cancer doesn't necessarily originate from a foreign agent like a virus, but can arise from genes normally present in cells. Varmus and Bishop were awarded the Nobel Prize for their work in 1989.

Today, there are scores of known oncogenes in humans - and the number continues to grow, as the scientific tools for uncovering them are further enhanced. Given their important role in initiating and maintaining tumor growth, oncogenes are considered prime targets for drug development.

Want to learn more?

You can learn more about oncogenes by reading stories that describe some recent discoveries by Broad Institute researchers and their colleagues. This research has helped identify genetic weak spots in a variety of cancers, including colon cancer and other cancer types, which could lay the foundation for new kinds of cancer therapies.

You can find out even more about cancer research at the Broad by reading about the Broad's Cancer and Chemical Biology Programs, which work on collaborative projects, including ones aimed at identifying new and improved cancer therapeutics.

In addition to the science happening now at the Broad, you can also check out an important page from the history book of science: the Nobel Prize in Physiology and Medicine 1989. In addition, Kimball's Biology Pages have some information about the Rous sarcoma virus and NOVA has a tutorial on how cancer grows.