GNAS and pseudohypoparathyroidism type 1A: Mutations involving exons 1–13 of the maternal GNAS allele underlie pseudohypoparathyroidism type 1A (PHP1A) in which patients develop resistance to PTH in the proximal renal tubules leading to elevated plasma PTH levels, hypocalcemia and hyperphosphatemia, and often resistance to multiple other hormones.