"The hair now is glowing green because green fluorescent protein is in the hair shaft," Robert Hoffman of San Diego-based AntiCancer, whose company led the study, said in a telephone interview.

"We saw lots of green fluorescent hair."

Writing in this week's issue of the Proceedings of the National Academy of Sciences , Hoffman and colleagues said their experiment involved skin taken from a mouse, treated, and then grafted to another mouse. They need to try it now directly on live mice.

"It's got a way to go before the market," said Hoffman, who worked with scientists at the Massachusetts Institute of Technology and Japan's Kitasato University School of Medicine.

But, he added: "We have a good system now to get genes into hair follicles."

Green fluorescent protein is often used by scientists testing genetic engineering techniques. It is a single gene that makes jellyfish glow green in the dark, is harmless to other animals and is easy to look for.

Hoffman said if one new gene functions in the hair follicle, others should.

His idea is to treat the hair loss that is caused by cancer chemotherapy. "You can imagine putting in genes that confer resistance to chemotherapy," he said.

Treating a receding hairline may be a bit more difficult, Hoffman said. "In male pattern baldness, the follicle makes a little fine hair instead of a normal hair shaft. This obviously involves a lot of genes."

Once the genes involved in colour and graying are better understood, it may even be possible to use gene transfer as a cosmetic procedure, he said.

The system could also be used for more standard gene therapy, Hoffman said. He believes a hair follicle could be genetically engineered to make any kind of protein.

"Why can't it make insulin? Why can't it make interferon (an immune system protein)?" he asked.

"You've seen people with two yards (metres) of hair. That is all protein. That little hair follicle is a tremendous factory."