The mini-mouse -- no relation to Mickey's girlfriend -- is a genetically altered rodent that grows to only half normal size because its cells contain a gene that produces an abnormal form of bovine (or cow) growth hormone.

The invention of the mini-mouse was a happy accident. Molecular biologists John J. Kopchick and Wen Y. Chen had first found that they could produce mega-mice -- which grew to twice normal size -- by giving mice an extra gene that contains the instructions for making normal bovine growth hormone. These "transgenic" mice continued to make their own growth hormone as well, and their bodies responded with supernormal growth.

Next, Kopchick and Chen tried for even bigger mice, by slightly altering the genetic code of the inserted cow gene. They predicted that the changes would improve the hormone molecule's shape and make it more active. Instead, the altered gene led to a hormone that suppressed normal growth, producing mini-mice.

Since the same modification should work in other species, Kopchick said it gives researchers a relatively simple way to make new kinds of miniature animals. The researchers now face the challenge of figuring out why such a small genetic change reversed the gene's effects.

The findings appear in today's Proceedings of the National Academy of Sciences.