PHP is classified in different subtypes, PHP-1A, pseudo-PHP, PHP-1B, PHP-1C and PHP-2, according to the presence or absence of AHO, the in vivo response to exogenous PTH and the in vitro activity of the α subunit of the stimulatory G protein (Gsα), a key element of the cAMP signalling pathway encoded by GNAS [1–4]. Here, GNAS is linked to pseudohypoparathyroidism type 1A.