Scientists have pinpointed genetic mutations which can cause a mole to become a deadly form of skin cancer.
Melanoma is becoming more common
In studies on zebrafish, they showed how a mutation to a gene called BRAF is critical to mole development.
And when combined with another gene mutation it can lead to a form of skin cancer called melanoma - cases of which have spiralled in recent years.
The study by Children's Hospital Boston and the Dana-Farber Cancer Institute is published in Current Biology.
Rates of skin cancer are rising faster in England than those for any other type.
Between 1991 and 2000, the number of cases of melanoma rose by 64% in men, and 45% in women, according to the Office for National Statistics.
Once melanoma has spread to other parts of the body it can be very difficult to treat.
Previous studies have indicated that the BRAF gene is mutated in about 75% of melanomas.
But its role in causing the cancer was unknown.
The researchers genetically engineered zebrafish to carry the mutated form of BRAF.
The mutant fish developed black-pigmented moles on their skin - but none developed melanoma.
However, when the fish were also made deficient in a separate gene called p53, which suppresses tumour growth, the moles developed into the cancer.
When cells from these tumours were injected into healthy zebrafish, they too developed melanomas. The next step will be to see how melanomas develop in the fish, and to look for other genes that may play a role in their development.
Researcher Dr Leonard Zon said: "Some of these genes may lead us to excellent pharmaceutical targets for treatment of melanomas."
Dr Catherine Harwood, consultant dermatologist at Cancer Research UK, said: "Melanoma is an important cancer and there is a rapidly increasing number of people who are getting it so any developments in understanding how and why it develops are important.
"This is an interesting paper and the preliminary data are exciting.
"However, we always have to be cautious in extrapolating directly from animal models to humans.
"For example, p53 appears to play a relatively small role in melanoma compared to other human skin cancers."
The researchers used zebrafish because its genes are very similar to humans.