Noncoding RNAs and Intranuclear Positioning in Monoallelic Gene Expression

Mammalian X inactivation, imprinting, and allelic exclusion are classic examples of monoallelic gene expression. Two emerging themes are thought to be critical for monoallelic expression: (1) noncoding, often antisense, transcription linked to differential chromatin marks on otherwise homologous alleles and (2) physical segregation of alleles to separate domains within the nucleus. Here, we highlight recent progress in identifying these phenomena as possible key regulatory mechanisms of monoallelic expression.