Non-coding RNAs (ncRNA) are RNAs that by definition do not code for proteins. They are estimated to make up ~98% of the human transcriptome. Once considered a transcriptional byproduct of "junk DNAs," ncRNAs have since been recognized as an important mechanism of gene regulation. In particular, small double-stranded RNAs (dsRNA) have been implicated in gene silencing through a mechanism commonly referred to as RNA interference (RNAi).