Small interfering RNA

Small interfering RNA (siRNA) is a class of double-stranded RNA molecules, 20-25 base pairs long. 
siRNA plays many roles, but it is most notable in the RNA interference (RNAi) pathway, where it interferes with the expression of certain genes. Genes are only affected if they have nucleotide sequences complementary to those of the siRNA. 
siRNA functions by breaking down mRNA after transcription. This prevents translation of the gene into protein. siRNA also acts in RNAi-related pathways, e.g., as an antiviral mechanism or in shaping the chromatin structure of a genome. The complexity of these pathways is only now being worked out.