Somewhere in Germany is a baby Superman, born in Berlin with bulging arm and leg muscles. Not yet 5, he can hold 7-pound weights with arms extended, something many adults cannot do. He has muscles twice the size of other children his age and half their body fat.

DNA testing showed why: The boy has a genetic mutation that increases muscle growth.

The discovery, reported in today’s New England Journal of Medicine, represents the first documented human case of such a mutation.

Many scientists believe the find could eventually lead to drugs for treating people with muscular dystrophy and other muscle-destroying conditions. And athletes would almost surely want to get their hands on such a drug and use it like steroids to bulk up.

The boy’s mutant DNA segment was found to block production of a protein called myostatin that limits muscle growth. The news comes seven years after researchers at Johns Hopkins University in Baltimore created buff “mighty mice” by “turning off” the gene that directs cells to produce myostatin.

“Now we can say that myostatin acts the same way in humans as in animals,” said the boy’s physician, Dr. Markus Schuelke, a professor in the child neurology department at Charite/University Medical Center Berlin. “We can apply that knowledge to humans, including trial therapies for muscular dystrophy.”

Given the huge potential market for such drugs, researchers at universities and pharmaceutical companies already are trying to find a way to limit the amount and activity of myostatin in the body. Wyeth has just begun human tests of a genetically engineered antibody designed to neutralize myostatin.

Muscular dystrophy is the world’s most common genetic disease. There is no cure and the most common form, Duchenne’s, usually kills before adulthood. The few treatments being tried to slow its progression have serious side effects.