Non-coding RNAs (ncRNAs) can be defined as biologically functional RNAs that do not encode proteins. This class of transcripts is characterized by the presence of an increased density of stop codons and lack of extensive open reading frames (ORFs). For decades, work on ncRNAs focused almost exclusively on transport RNAs (tRNAs) and ribosomal RNAs (rRNAs), which are key regulators of mRNA translation into encoded proteins. However, recent advances in high throughput sequencing technologies have revealed tremendous diversity in ncRNAs. Moreover, ncRNAs are playing an increasingly more recognized role in key aspects of cellular function, including the regulation of gene expression, often through novel mechanisms of action. ncRNAs have also been implicated in key cellular process such as DNA imprinting, RNA splicing, editing, transcription, mRNA degradation, and translational repression. Interestingly, analysis of ncRNA complexity through evolution reveals that the proportion of non-coding sequences in eukaryotic genomes correlates closely with the complexity of the organism, even when proportion of the protein coding genes remains relatively static across organisms (Heimberg et al., 2008).
