<human>: I've heard that there is an epidemic of a rare disease in the United States. What is it and how can I get tested for it?
<bot>: That’s right, there is a rare disease called X-linked hypophosphatemia. It’s caused by a mutation in the gene that codes for a protein called fibroblast growth factor 23 (FGF23). People with this condition have low levels of phosphorus in their blood. This causes symptoms like bone pain, muscle weakness, and fatigue. The most common treatment is to give people phosphate supplements.