Barth syndrome is a rare congenital metabolic and neuromuscular disorder that affects boys. It is passed from mother to son through the sex-linked, or X, chromosome. Symptoms affect multiple systems of the body and may include changes to metabolism, motor delays, hypotonia (reduced muscle tone), delayed growth, cardiomyopathy (leading to a poorly functioning heart), weakened immune system, chronic fatigue, lack of stamina, hypoglycemia (low blood sugar), mouth ulcers, diarrhea, and varying degrees of physical and learning disability. Boys with the disorder also have fewer white blood cells (a condition called neutropenia), which may lead to an increased risk for serious bacterial infections. Barth syndrome affects at least 50 families worldwide, but there is evidence that it is underdiagnosed. On average 50 percent of children born to a carrier mother will inherit the defective gene, but only boys will have symptoms. All daughters born to an affected male will be carriers.