GNAS and pseudohypoparathyroidism type 1A: Hypocalcemia develops also in the different variants of pseudohypoparathyroidism (PHP), i.e., disorders with PTH-resistance that are caused by inactivating maternal GNAS mutations involving the exons encoding the stimulatory G protein (Gsα) (PHP type Ia; PHP1A) or by methylation changes on the maternal GNAS allele that reduce Gsα expression (PHP type Ib; PHP1B) (17, 18).