In about 80% of affected families, heterozygous loss of function mutations in the Gs alpha gene are detected. It is of interest mutations inherited from the mother always lead to the complete form of the disorder, that is PHP; on the contrary when the same mutations are inherited from the father, patients show the physical abnormalities of Albright’s Osteodystrophy, without any evidence of hormone resistance. This pattern of inheritance is consistent with a tissue-specific paternal imprinting of the Gs alpha gene. Imprinting is an epigenetic phenomenon by which one of the 2 alleles undergoes partial or total loss of expression; in the case of the Gs alpha gene one would expect that only the paternal allele should be lost in specific endocrine tissues, such as the kidney, the thyroid and the gonad, which are the target organs resistant to hormone action in PHP Ia. Indeed, our group demonstrated that in specific human endocrine tissues also Gs alpha transcription mainly derives from the maternal allele (Mantovani et al., J Clin Endocrinol Metab, 2002. 87: 4736-4740). In particular a predominant maternal origin of transcription was found in thyroid and gonad and these data are consistent with the clinical finding of TSH and gonadotropin resistance present in patients affected with PHP. Interestingly, we observed a predominance of the maternal allele also in the pituitary gland, an organ which is not classically included among the target organs resistant to hormone action in PHP Ia.