Noncoding RNA molecules.

SIGNIFICANCE: The existence of noncoding RNAs (ncRNAs) has been known since the 1960s, but it was not until the last decade of the twentieth century that their significance and functions began to be understood. Since then there has been a rapid expansion of our knowledge and understanding of ncRNAs, in particular small RNAs. Although initially less familiar than mRNA, tRNA, and rRNA, ncRNAs play crucial roles—including some that have not yet been elucidated—in normal cellular functions. The many ncRNAs so far discovered have roles in DNA replication and chromatin structure, processing of other RNAs, and transcriptional and post-transcriptional control of gene expression, genome integrity, and mRNA stability.