U.S. government scientists saved two men dying of melanoma by genetically altering their own white blood cells to attack their tumours, in what's being called the first major success in battling cancer with gene therapy.

While the men appear disease-free almost two years after the experimental therapy, it wasn't a panacea. Fifteen other people with melanoma weren't helped by similar treatments, so the U.S. National Cancer Institute is trying to strengthen the shots.

Still, specialists proclaimed the work, published online Thursday by the journal Science, as an important advance — gene therapy with the potential to fight cancer's worst stage, when it has spread through the body. 

The institute hopes to begin testing it soon against cancers more common than melanoma, such as advanced breast or colon cancer.

Doctors can't predict how the two men will fare long-term.

An estimated 4,500 Canadians will be diagnosed this year with malignant melanoma, and almost 900 will die, according to the Canadian Cancer Society. Melanoma is notorious for returning years after patients think they've subdued it.

"I'm cured for now," is how a grateful Mark Origer, 53, of Watertown, Wis., put it after a checkup from NCI doctors this week.

The approach remains highly experimental, requiring years of additional research.

"Clearly this is a first step," cautioned Dr. Len Lichtenfeld of the American Cancer Society. "We have to be very cautious about not raising hopes too much."

But, "it is exciting," he added. "It certainly is a proof of concept that this approach will work."

"It's one of the first documented, effective cases of cancer gene therapy working," added Dr. Patrick Hwu, melanoma chairman at the University of Texas M.D. Anderson Cancer Center, who once worked with the NCI team.