Small double-stranded, non-protein coding RNAs (21-31 nucleotides) involved in gene silencing functions, especially rna interference (RNAi). Endogenously, siRNAs are generated from dsRNAs (rna, double-stranded) by the same ribonuclease, Dicer, that generates miRNAs (micrornas). The perfect match of the siRNAs' antisense strand to their target RNAs mediates RNAi by siRNA-guided rna cleavage. siRNAs fall into different classes including trans-acting siRNA (tasiRNA), repeat-associated RNA (rasiRNA), small-scan RNA (scnRNA), and Piwi protein-interacting RNA (piRNA) and have different specific gene silencing functions.