A boy was born with twice the normal muscle

Physicians at Charité - Universitätsmedizin Berlin examined a newborn boy with visibly bulging muscles and unusually low body fat, and traced the cause to a mutation disabling both of his copies of the myostatin gene.

Myostatin normally limits muscle growth, and without any working copies of the gene, the boy's muscles grew roughly twice as large as typical for his age, a condition confirmed through genetic testing of the boy and his mother. His mother, who carried one mutated copy of the gene, was a former professional athlete.

The case offered rare direct evidence, in a person rather than an animal, that disabling myostatin could produce the extreme muscle growth already seen in mice and cattle bred with similar mutations. At the time of the report the boy was healthy, and long-term effects of growing up with disabled myostatin, including on the heart, were still unknown. It remains a single case report.

One disabled gene, and his muscles grew to match.