GNAS and pseudohypoparathyroidism type 1A: PHP is caused by mutations in either GNAS, STX16, or GNASAS1 on the maternal allele of chromosome 20q13 and is subdivided into groups depending on the clinical and hormonal phenotypes: type 1a (known as Albright Hereditary Osteodystrophy, AHO; OMIM #103580), type 1b (OMIM #603233), type 1c (OMIM#612464), and type 2 (OMIM #203390).