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A few years ago, Dr. Ellen Heber-Katz, an immunologist at the Wistar Institute in Philadelphia, was conducting an experiment with those mice, which develop a disease similar to lupus. As is common, Dr. Heber-Katz punched a pattern of holes in each mouse's ear to so she could tell which mouse was which.

Three weeks later, she said, when she checked on the experiment, "there were no ear holes." Dr. Heber-Katz could not tell the mice apart. That ruined her experiment but sent her into a whole new field of study.

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She has since found that those mice, unlike other mice, can also regrow parts of their hearts, tails and spinal cords. The fact that the "healer mice" do not form scars appears to be important, she said. Scars block the ability of cells to divide and block signals from the epidermis, a layer of skin, that appear necessary to begin regeneration, she said.

Scientists have been fascinated by regeneration since the 1700's, when a French scientist, René-Antoine Ferchault de Réaumur, for whom a temperature scale is named, noticed at the fish market that some crayfish had legs that were not the same size. He surmised that some legs had grown back after having been cut off. Other scientists around the same time found that hydra, snails and other creatures could regenerate, setting off a frenzy of experimentation.

"Almost everything that moved in Europe was amputated," said Dr. Alejandro Sánchez Alvarado, an associate professor of neurobiology and anatomy at the University of Utah. Even Voltaire decapitated a snail to see its head grow back. He then wrote to friend who was blind that he hoped that a similar process could be harnessed in people.

But that has proved difficult. Dr. Thomas Hunt Morgan studied regeneration at the beginning of the 20th century and basically declared the subject intractable. Dr. Morgan himself moved on to other areas, studying genes and chromosomes in the fruit fly, work that won him a Nobel Prize as the father of modern genetics.

Now, scientists hope to understand regeneration by finding the genes involved. Dr. Heber-Katz has found at least seven locations on the chromosomes of her "healer mice" that appear to contain genes that correlate with the ability to regenerate.

Dr. Sánchez Alvarado is systematically turning off genes in planaria in hope of discovering which are necessary for regeneration. Dr. David Stocum, a biology professor and the dean of the School of Science at Indiana University-Purdue University Indianapolis, is studying which genes are active in tadpoles, which can regrow limbs, but not in adult frogs, which cannot.

One gene that appears to be important is msx1. It helps keep cells in an embryo from dividing prematurely. Dr. Keating found that when the gene was turned on in mouse muscle cells that were growing in culture, the cells de-differentiated into stem cells. Another tantalizing clue of the importance of msx1, Dr. Keating said, is that the gene is turned off in people, except in the fingertips, the one part of the body where regeneration has been seen.

Dr. Keating has also made mouse muscle cells de-differentiate by using an uncharacterized mixture of proteins extracted from a newt. Scientists at the Scripps Research Institute did that with a chemical called myoseverin.

The scientists focusing on natural regeneration concede that stem cell scientists are highly likely to make more progress in the near future.

"It's a lot easier to take a stem cell and put it back somewhere than to figure out what's going on here," Dr. Heber-Katz said.

But the salamander types insist that their time will come.

"Given that these mechanisms work so well in animals that are built like us," Dr. Brockes said, "it would really surprise me if there wasn't some role for this strategy in the future."