IL6 and Barth syndrome: Since BTHS patients are exercise intolerant, there are two possibilities: 1) that normal daily activities in the BTHS patients mimic intense exercise in the muscles of healthy controls, i.e. normal activity increases circulating IL-6 in Barth Syndrome, or 2) that IL-6 is being secreted at higher levels by immune cells (possibly dysfunctional neutrophils) in Barth Syndrome patients relative to healthy controls.