For example, gain-of-function mutations in the region of the immunoglobulin domain of fibroblast growth factor receptor 1 (FGFR1) result in Pfeiffer syndrome and osteoglophonic dysplasia, whereas loss-of-function mutations throughout the protein cause hypogonadotrophic hypogonadism [10,11]. Here, FGFR1 is linked to hypogonadotropic hypogonadism.