Complex structures of imprinted gene loci. (a) Genetic map of the 2-Mb imprinted domain in chromosome 15q11-q13 associated with PWS and AS. An IC is associated with the 5′ end of the bicistronic SNURF-SNRPN locus. Blue or pink circles, imprinted genes showing expression of only the paternal or maternal allele, respectively; open squares, nonimprinted genes; small arrows, overlapping and antisense transcription; large arrows, regional imprint control through an IC; zigzag lines, common breakpoint (BP) regions for deletions. (b) Genetic map of the 1-Mb imprinted domain in chromosome 11p15 associated with BWS. Symbols are the same as in a; colored squares represent genes imprinted in some tissues in mice, but not yet shown in humans. Three loci have been specifically implicated in the pathogenesis of BWS. The 5′ H19 region appears to act as a local IC regulating imprinting at IGF2, whereas a 5′ CpG-island for LIT1 (KvLQT1-AS) has been proposed to perhaps (denoted by ?) act as an IC for either CDKN1C, IGF2, and/or other genes in the 11p15 domain. (c) Genetic map of the 75-kb GNAS1 locus in chromosome 20q13. Three alternative promoters of transcription all lead to splicing to a common set of 12 downstream exons. Each exon 1 region has a different pattern of parental origin–specific allelic methylation and transcription, and each leads to a different encoded protein product.