The main classes of small ncRNA are short interfering RNAs (siRNAs), microRNAs (miRNAs) and PIWI-interacting RNAs (piRNAs). siRNAs are generated by cleavage of exogenous long double-stranded RNA precursors in response to viral infection or artificial introduction. In contrast, miRNAs are generated from endogenous transcripts containing stem-loop structures.
