A cancer gene assumed to be foreign
Researchers identified src as the first confirmed oncogene in 1970, working with a cancer-causing chicken virus and showing, through work led by G. Steve Martin at the University of California, Berkeley, that the gene actively drove tumour formation once it infected a cell. For years the working assumption was that oncogenes like src were essentially foreign, viral material, genetic invaders that hijacked a cell’s machinery and turned it cancerous from the outside. That assumption held until 1976, when Dominique Stéhelin, working with J. Michael Bishop and Harold Varmus at the University of California, San Francisco, found something that reframed the entire picture: normal, healthy cells, including in humans, already carried a very similar version of the src gene, meaning the virus had not invented a cancer-causing gene at all but had captured and slightly altered one that was already part of ordinary cellular machinery.
The gene was already there
The method behind the 1976 discovery involved comparing the viral version of src against the genetic material of normal, uninfected cells across a range of species, looking for a matching sequence that would reveal whether the cancer-causing gene had a cellular origin. Finding a closely related gene, now called a proto-oncogene, in ordinary chicken cells and beyond meant the difference between a cancer-causing gene and a normal one was not simply presence versus absence but a matter of mutation, mis-regulation or abnormal activity level in a gene the cell was always going to have anyway. This reframing gave cancer biology a specific, testable model to build on: identify the normal function a proto-oncogene performs, usually related to controlling cell growth and division, and then work out exactly what change turns that same gene dangerous.
From a chicken virus to a Nobel Prize
The proto-oncogene model has held up as the foundation of modern cancer genetics, extended well beyond the original chicken virus src gene it was built on. Bishop and Varmus received the 1989 Nobel Prize in Physiology or Medicine specifically for establishing the cellular origin of retroviral oncogenes, and the same basic logic, a normal gene that can be pushed into a cancer-causing state through mutation, has since been confirmed across dozens of other genes and cancer types. Robert Weinberg’s 1982 identification of the first human oncogene, a mutated version of a gene called HRAS found in bladder cancer cells and characterised in detail by Mariano Barbacid, confirmed that the same mechanism responsible for a chicken tumour virus applied directly to human cancer, closing the gap between an animal virus model and a genuinely human disease process.
The same mechanism in human cancer
What the oncogene discovery did not provide, on its own, was a complete account of how cancer actually develops in a person over time. A single mutated proto-oncogene is generally not sufficient by itself to produce a malignant tumour; the broader multi-hit model that emerged from this research holds that cancer typically requires several independent genetic changes accumulating in the same cell line before malignancy results, meaning the src story explains one necessary kind of event rather than the entire disease process. The Src protein itself, now understood as a tyrosine kinase that helps control cell survival, blood vessel formation and cell movement, shows elevated activity in roughly half of tumours studied across colon, liver, lung, breast and pancreatic tissue, with activity in some premalignant colon polyps measured at five to eight times normal levels, but that elevated activity is one contributing factor among several rather than a sole cause.
One piece of a multi-step disease
The practical consequence of recognising cancer-causing genes as mutated versions of normal genes, rather than foreign invaders, reshaped both cancer research and eventually cancer treatment. Once researchers could point to a specific gene and a specific abnormal protein driving a tumour’s growth, it became possible to design drugs targeting that protein directly, an approach now central to an entire category of cancer therapies aimed at particular molecular abnormalities rather than at rapidly dividing cells in general. The idea that a virus could accidentally reveal a fundamental piece of normal human biology, in this case a gene controlling ordinary cell growth, by hijacking and slightly altering it, also became a template researchers have used to identify other cancer-related genes since, well beyond src and HRAS specifically.
From foreign invader to targetable protein
This is worth the time for how cleanly it reframes a basic question about cancer: not what foreign thing causes it, but what goes wrong with the cell’s own existing machinery. The 1976 discovery is a genuinely elegant piece of scientific reasoning, using comparison across species to trace a viral gene back to its ordinary cellular ancestor, and it rewards attention to that comparative method rather than treating the conclusion as simply handed down. Readers should not expect this one discovery to explain cancer fully; the multi-hit model it fed into makes clear that a single altered proto-oncogene is a piece of a larger, multi-step process rather than a complete explanation. As an origin point for how modern cancer genetics understands the disease, though, it is close to essential.