Organization and expression of GNAS1. Four first exons (distinct promoters) are alternatively spliced to exons 2–13 that are shared by all four transcripts. The NESP55 and Gsα transcripts are expressed from the maternal allele, while the XLαs and exon 1A transcripts display paternal expression. Note that imprinting of GNAS1 is tissue-specific and that the pattern of allelic expression shown here represents that seen in the pituitary. Lollipops correspond to CpG dinucleotides where the C is subject to modification by the addition of a methyl group at the 5′ position. A filled lollipop indicates methylation of a given CpG site, while an open lollipop indicates that the corresponding site on the other allele is not modified. RT-PCR using a primer () in exon 10 generates a product that spans a polymorphism (*) in exon 5 allows the assessment of allelic expression of all four transcripts in pituitary. RT-PCR with the exon 10–specific primer also permits the assay of sites of mutation in exons 8 and 9 (+) for the analysis of Gsα imprinting in pituitary somatotroph tumors.