Melanoma, the most dangerous form of skin cancer, can be held in check by a natural immune system molecule, a study has shown.
Scientists believe the research, in mice, has important implications for treating malignant and deadly skin cancer.
Mice with melanoma were treated with white blood cells that produce the molecule, interleukin-9.
The animals showed a "profound" resistance to tumour growth.
Interleukin-9 is a "cytokine", a type of cell-signalling molecule that plays a role in the immune system.
The cells that produce it, called TH9 cells, have been found in both normal human blood and skin.
But they are either absent or present at very low levels in human melanoma tissue.
The new research is reported in the latest issue of the journal Nature Medicine.
Lead scientist Dr Thomas Kupper, from Brigham and Women's Hospital in Boston, US, said: "Immunotherapy of cancer is coming of age, and there have been exciting recent results in patients with melanoma treated with drugs that stimulate the immune system.
"We hope that our results will also translate to the treatment of melanoma patients, but much work still needs to be done."
Each year around 12,000 people in the UK are diagnosed with melanoma and 2,000 die from the disease.