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They're six healthy Tasmanian devils, seemingly identical to the rest of their kind. Yet they're being kept at a secret location in Tasmania, and scientists are calling them the Special Six. What makes these devils different, and has their keepers so hopeful, is their genetic makeup. A virulently infectious cancer has wiped out more than half of their marsupial species, and so far only one devil has shown signs of resisting the disease. Starting this week, the Special Six will be injected with tumor cells to see if they can do the same. If their immune systems attack the cells, they may have to be renamed, says Greg Woods, the immunologist heading the research: "They'll be the Super Six."

Since hideous facial lesions were first spotted on Tasmanian devils in 1996, the species has been in freefall. At current rates, it's predicted that one of Australia's most unusual animals could vanish from the wild within three years. Spread by biting during mating and one of only three communicable cancers ever seen, devil facial tumor disease has baffled scientists. And as it rages through 60% of the devils' habitat, introduced pests like feral cats and foxes have been taking the place of Tasmania's largest native predator.

Sydney University geneticist Kathy Belov admits that until recently she thought the species, soon to be officially listed as endangered, was doomed. Her team's work on major histocompatibility complex (MHC) genes — which activate an immune response in all vertebrates — confirmed late last year that devils have so little genetic diversity that their immune systems simply don't treat the tumor cells as a threat. That was the grim news from the island's east, where the disease has hit hardest. But it now seems that devils from the more isolated west could be different. It's a distinction that just might save them — and their species. "If you'd asked me six months ago if there was much hope for the devil I would have said it's not looking good," says Belov. "I'm now much more hopeful."

Good news came first from a devil named Cedric. When tests showed that he had a different MHC type from eastern devils, Cedric was injected with dead tumor cells. The researchers were elated when his body began fighting them. Last December he was injected with live tumor cells. So far, he's still healthy. The Special Six, caught last year, also have MHC types not seen in the east, says immunologist Woods, who's based at the University of Tasmania. "It's never happened before that a disease has spread through a population without some animals being resistant — even Ebola doesn't kill everyone." So naturally resistant devils ought to exist, he says. "This tumor has broken all the rules so far. It has to obey one eventually."

One problem is that no one knows how long the incubation period for facial tumor disease is. But every week that passes without Cedric falling sick makes Woods sleep better: "I'm quite confident now that he's immune." Hamish McCallum, the University of Tasmania ecologist coordinating the scientific effort to save the species, is hopeful too, but "we're not in any position, on the basis of one individual, to say we know what's going on," he says. "It's too early to unfurl the 'mission accomplished' banner." If Cedric or any of the Special Six do prove resistant, they could be bred back into the wild and into captive populations, improving the species' immunological fitness.

In the island's northwest, one of McCallum's students has been tracking an odd trend in the epidemic. In the past two years, only 10 animals with tumors have been found in a forestry area known as West Pencil Pine, a favorite devil haunt. That suggests that the disease's spread is slowing. Could local devils be resisting it? Belov and Woods are both itching to compare MHC types in the region, and a major effort to trap West Pencil Pine devils will soon begin. "The next month could be a watershed," says Woods. "We're hoping for a lot of positive data."

As the genetic detective work goes on, "Noah's arks" of devils are being stocked. Already 73 are in Australian zoos and wildlife parks, while another 63 have been caught this year. The aim is to build up a captive population of 1,500 — but that, says Steven Smith, manager of the state's Save the Tasmanian Devil program, may require the funds and facilities of a massive international breeding program, the first for an Australian species. Keeping devils is costly, as adults must be held in large individual enclosures to prevent fighting. Extra money is coming — $10 million from the federal government over the next five years — but more is needed, says Belov: "I don't think there's enough urgency attached to this." She has 600 tissue samples she can't afford to analyze and relies on volunteers and students for lab work. She's also one of a number of scientists seeking financial backing to sequence the devil genome for more clues.

Meanwhile, proposals to fence off Tasmanian peninsulas and establish devil colonies on nearby islands are about to be revisited, though some ecologists oppose using islands because of the damage an experienced new predator could do. Cordoning off peninsulas would also be expensive to maintain and hard to patrol. "Fencing out a disease is much harder than fencing out a predator," says Hamish McCallum. But like others, he worries about relying on captive devils, which will inevitably lose some wild traits: "I wouldn't want all my eggs in that basket."

Why not reintroduce devils to the Australian mainland, where, after all, they once roamed? Smith says the Save the Devil steering committee has informally discussed that; he thinks "it's a good idea to find out more about it." Those who oppose ecological tinkering are unlikely to support such a bold experiment. Of course, if Cedric or any of the Special Six stay well, it could be the need for such measures, rath-er than their species, that disappears.

• Lisa Clausen