Somewhere in Germany is a baby Superman, born in Berlin with bulging arm and leg muscles. Not yet 5, he can hold seven-pound weights with arms extended, something many adults cannot do. He has muscles twice the size of other kids his age and half their body fat. DNA testing showed why: The boy has a genetic mutation that boosts muscle growth. The discovery, reported in Thursday'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...