of TSH, LH, or isoproterenol, thus explaining the isolated PTH resistance. The selective effect of this mutation on PTH signaling, however, could not be verified in a subsequent study [70], and it is possible that the discrepancy between the two studies stems from the use of different cell types and/or assays; the second study used mouse embryonic fibroblasts null for endogenous Gsα [70, 71]. Alternatively, the three patients may have PHP-Ia, consistent with the observation that two of them exhibited advanced bone age [154], which is a typical sign of AHO. Since the urinary cAMP response to exogenously administered PTH is blunted in PHP-Ib patients [131], defects in the gene encoding PTHR1 seemed like a good candidate at the time. However, several studies have ruled out this possibility [153, 155-157].
