Complex sentence: An American boy born in 2005 (Liam Hoekstra) was diagnosed with a clinically similar condition but with a somewhat different cause: his body produces a functional myostatin, but a defect in his myostatin receptor makes his muscles not respond to the myostatin signal.
Split into simpler sentences: An American boy born in 2005 (Liam Hoekstra) was diagnosed with a clinically similar condition but with a somewhat different cause. In contrast to the first case, this boy produces a functional myostatin, however he has a defect in his myostatin receptor so that his muscles do not respond to the myostatin signal.