On the basis of their length and function this heterogeneous group of transcripts can be further categorized into short ncRNAs and long ncRNAs. The short ncRNAs include microRNAs (miRNAs) and short-interfering RNAs (siRNAs), which are generally 18–25 nucleotides in length. Small RNAs (smRNAs) such as the small nucleolar RNAs (snoRNAs), smRNAs, piwi-interacting RNA (piRNAs) are between 20 and 300 nucleotides in length. The long ncRNAs (>200 nucleotides) can further be grouped into different categories on the basis of their origin (Orom and Shiekhattar, 2011) and have a broader spectrum of functions including chromatin modulation, recruitment of transcription factors, and nuclear-cytoplasmic transport (Mattick and Makunin, 2006). Indeed, there is now compelling computational and experimental evidence that many previously uncharacterized ncRNAs of unknown function, considered largely “transcriptional noise,” have key functional roles in almost all aspects of cellular physiology.
