Simplified scheme of the imprinted murine <i>Gnas</i> locus.

<p>The differential, parental allele-specific expression of the various transcripts of the locus is indicated by arrows. Open and filled boxes represent non-coding and coding exons, respectively. Spliced transcripts and encoded proteins are only shown for <i>Gnas</i> and <i>Gnasxl</i> above and below the parental alleles. On the paternal allele <i>Gnas</i> expression is silenced in specific tissues (shaded <i>Gnas</i> exon 1 box). <i>Gnasxl</i> splices in frame onto exon 2 and encodes an ‘extra large’ α-subunit. Splicing onto exon N1 is limited to neural tissue, results in premature termination of the open reading frame and expression of a truncated XLN1 protein. The alternatively spliced exon A20 is found in a minority of <i>Gnasxl</i> transcripts, mainly in combination with exon N1, and introduces a frameshift and early termination codon in exon 2. <i>Nespas</i> and <i>exon 1A</i> promoters generate non-coding regulatory RNAs. The <i>Nesp</i> transcript splices onto exon 2 and has a regulatory as well as protein coding function. Regions of differential DNA methylation, including the imprinting control region (ICR) at <i>Nespas</i>, are symbolized by M.