The chromosome 15 imprinting centre (IC) region has undergone multiple duplication events and contains an upstream exon of SNRPN that is deleted in all Angelman syndrome patients with an IC microdeletion.

Imprinting of the Prader-Willi/Angelman syndrome region on human chromosome 15 is regulated by an imprinting centre (IC), which spans 5' exons of the gene encoding the small nuclear ribonucleoprotein N ( SNRPN ). The IC/ SNRPN transcripts are initiated at two alternative start sites, which share a high degree of sequence similarity with each other and with two newly identified sites 63 and >700 kb further upstream. Three of these sites are hypermethylated on the maternal chromosome, whereas one displays an oppositemethylation pattern. We have also identified novel splice variants of the IC/ SNRPN transcripts and hitherto undetected exons. One of these exons, which we designate u5, is deleted in all Angelman syndromepatients with a microdeletion of the IC. We conclude that elements of the IC region have undergone multiple duplication events and that u5 or a sequence close by may play a role in maternal imprinting.