Imprinted noncoding RNAs.

Imprinted genes are silenced in a parental-specific manner and tend to occur in clusters. All well-characterised imprinted clusters contain noncoding RNAs that are silenced according to parental origin. These can be broadly classified into long noncoding RNAs and short regulatory RNAs. Functional testing has shown that long noncoding RNAs can be crucial imprinting elements and act in cis throughout the cluster to silence protein-coding genes. Whether silencing occurs via transcription of the noncoding RNA or the actual transcript is not clear. The short regulatory RNAs, both small nucleolar RNAs and microRNAs, act in trans, generally outside the cluster from which they arise. As these RNAs are expressed according to parental origin, the regulation of their targets is also parental-specific. We review knowledge of imprinted noncoding RNAs and models for how they function.