Melanoma is a particularly aggressive form of cancer
Scientists have pinpointed a genetic mutation which may trigger up to 70% of cases of melanoma, the most deadly form of skin cancer.
It was previously known that the BRAF gene was often damaged or mutated in melanomas - but not whether this was a cause or result of the cancer.
The latest study shows acquiring the mutation can be the first in a cascade of genetic changes leading to melanoma.
The Institute of Cancer Research study features in the journal Cancer Cell.
The most serious form of skin cancer
Sun exposure is the main - and most preventable - risk factor, causing genetic damage to the skin
Around one third of melanomas develop from normal moles
The rest develop on areas of previously normal skin
Warning signs include:
Two halves of a mole do not look the same
The edges of the mole are irregular, blurred or jagged
Colour is uneven, with more than one shade
Mole is wider than 6mm
It confirms that BRAF is a driving force behind the disease and could be the trigger that leads to skin cancer.
Although the study was carried out in mice, the researchers said that melanomas which develop in the animals closely resemble those that develop in humans.
Lead researcher Professor Richard Marais said: "We know that excessive sun exposure is the main cause of skin cancer, but not much is known about the genetics behind it.
"Our study shows that the genetic damage of BRAF is the first step in skin cancer development.
"Understanding this process will help us develop more effective treatments for the disease."
There are around 9,500 new cases of malignant melanoma and more than 2,300 deaths from the disease each year in the UK.
Over-exposure to sunlight causes at least two thirds of all malignant melanomas.
REDUCING THE RISK
Spend time in the shade between 1100 and 1500
Make sure you never burn
Wear a t-shirt, hat and sunglasses
Take extra care with children
Use factor 15+ sunscreen
This excessive exposure damages DNA and causes genetic mutations.
Dr Lesley Walker, of Cancer Research UK, said: "There is lots of exciting research focused on developing new therapies that will block the function of mutant BRAF.
"A better understanding of the genetics of skin cancer can help scientists develop more targeted drugs with fewer side effects to treat the disease."