About five years ago, Ellen Heber-Katz, an immunologist at the Wistar Institute in Philadephia, sent one of her students to punch holes in the ears of a group of experimental mice. Three weeks later, however, all the holes had diappeared. When the two researchers tried again, the same thing happened. Dr Heber Katz nicknamed the mice, a strain she was using to study multiple sclerosis, "healer mice". Her subsequent investigation showed that healer mice can regrow the tips of their tails if these are cut off.