RNAi pathways are guided by small RNAs that include
'''SiRNAsiRNA:'''
* Small interfering RNA (siRNA), sometimes known as short interfering RNA or silencing RNA, is a class of 20-25 nucleotide-long double-stranded RNA molecules.
* SiRNAs can also be exogenously (artificially) introduced into cells by various transfection methods to bring about the specific knockdown of a gene of interest. [http://en.wikipedia.org/wiki/SiRNA Source]