The moment the little boy was born, the hospital staff knew there was something unusual about him. His muscles looked nothing like the soft baby muscles of the other infants in the nursery. They were bulging and well defined, especially in his thighs and upper arms." Everybody noticed," said Markus Schuelke, a pediatric neurologist at Charite University Medical Center in Berlin. The baby, it turned out, had a rare double dose of a genetic mutation that causes immense strength in mice and cattle...

The boy's story, written by Schuelke, appears Thursday in The New England Journal of Medicine ["Myostatin Mutation Associated with Gross Muscle Hypertrophy in a Child," by Markus Schuelke, M.D. et al]. At the baby's birth, Schuelke said, his doctors were worried. The infant was jittery, jerking his limbs, much the way people sometimes involuntarily jerk their legs when they are falling asleep. "At first we thought it might be epilepsy," Schuelke said.

After two months, the jerking movements had subsided, but the puzzle of the baby's muscles remained. Then Schuelke had an idea. He knew that Se-Jin Lee at Johns Hopkins University, working with mice, had found that when both copies of a gene for a protein called myostatin were inactive the animals grew up lean and so muscular that Lee called them "mighty mice".

It turned out that cattle breeders, decades ago, had stumbled upon the same genetic trick, developing a strain known as Belgian Blue, or double-muscle cattle. The cattle are hefty, meaty, extremely muscular and lean, and they, too, researchers later found, had inactive myostatin genes.

"We had a big discussion about what to do," Schuelke said. "We remembered the mighty mice and the Belgian Blue cattle. This child looked like that." The child's mother was strong - she had been a professional sprinter in the 100-meter dash - and she came from a strong family. Her grandfather, a construction worker, had unloaded curbstones by hand, hefting stones weighing at least 330 pounds. There was no information on the baby's father.

Some researchers are trying to turn off the myostatin gene in chickens to produce more meat per bird. And several breeds of cattle have natural variations in the gene that, aided by selective breeding, give them far more muscle and less fat than other steer.