FIHP may be caused by germline mutations in MEN1 [10, 11], CDC73, calcium-sensing receptor (CASR; OMIM: 601199) [12], and, as reported more recently, glial cells missing transcription factor 2 (GCM2 OMIM: 603716) and familial hypocalciuric hypercalcemia type 1 (FHH1). Here, MEN1 is linked to familial isolated hyperparathyroidism.