Given its efficacy in tumor-induced osteomalacia and X-linked hypophosphatemic rickets, two related disorders of FGF23-mediated hypophosphatemia, we explored treatment with burosumab in a young adult with CSHS.<h4>Methods</h4>In this open-label, single-patient trial conducted in the clinical research unit of an academic children's hospital, burosumab was administered subcutaneously every 4 weeks for 3 years. The gene discussed is FGF23; the disease is X-linked hypophosphatemic rickets.