GNAS and pseudohypoparathyroidism type 1A: Another related disorder is PHP1c, which is identical to PHP1a in terms of the presence of AHO and hormone resistance, but in contrast to PHP1a, in vitro assessment of Gs-alpha protein activity, using solubilized Gs-alpha from patient derived erythrocyte membranes, reveals no abnormality, and mutations of GNAS are usually not observed [Mantovani et al., 2006; Thiele et al., 2011].