It's a genetic condition that has only been diagnosed in a handful of people (30+ total, ever), but is believed to be rather under-diagnosed (Frequency should be about 1/200,000 - still very rare but not a 1 in 50 million condition). It affects motor skills, development, stamina, metabolism, the immune system, and other things.

Wikipedia gives the cause of Barth Syndrome as:
"Mutations in the BTHS gene, tafazzin (TAZ), are associated with cardiolipin molecules in the electron transport chain and the mitochondrial membrane structure. The gene is 6,234 bases in length, mRNA of 879 nucleotides, 11 exons/10 introns, and amino acid sequence of 292 with a weight of 33.5 kDa. It is located at Xq28;[4] the long arm of the X chromosome. Barth Syndrome is caused by 60% frameshift, stop, or splice-site alterations and 30% change in protein's charge. Barth syndrome is found exclusively in males."

So, a specific mutation of a gene as noted above.

Barth syndrome is a rare, sex-linked genetic disorder of lipid metabolism that affects males. Typically, boys with Barth syndrome present with hypotonia (low muscle tone) and dilated cardiomyopathy (labored breathing, poor appetite, and/or slow weight gain) at or within the first few months after birth.

The Barth Syndrome Foundation, together with our affiliates, is a community of families, physicians, scientists, donors and volunteers around the world.

As our mission statement says, we are dedicated to saving lives through education, advances in treatment, and finding a cure for Barth syndrome - a sometimes fatal, oftentimes debilitating genetic disease.