Cyrsvita is orphan drug and can be used in children over one year of age and adolescents with skeletons. The medicine is used to treat XLH when evidence of bone disease has been obtained radiographically. XLH is a hereditary disease linked to the X chromosome that is characterized by a low phosphate blood level. However, phosphate is essential for the construction of teeth and bones and contributes to their maintenance of strength. If the mineral is missing, the ratio of bone, calcium and phosphate is disturbed. It comes to the softness of the bones and a bending of the lower extremities, which shows up in X- or O-legs. Those affected develop rickets. The cause of XLH is a genetic mutation in the PHEX gene that results in increased fibroblast growth factor-23 (FGF23) hormone production. The FGF23 formed in the bone cells regulates vitamin D and phosphate balance. If the FGF23 concentration increases as a result of the genetic defect, there is a disruption of phosphate reabsorption and increased excretion via the urine. The phosphate level drops and leads to a mineralization disorder of the bone.