In earlier sections, we saw that introducing mutations or new DNA sequences into the genome and observing the resulting phenotypes could provide information about the function of the altered or introduced DNA. We could also analyze gene function by temporarily turning a gene off and observing the effect of the absence of the gene product on the phenotype. For many years, there was no such method for selectively affecting gene expression. However, the discovery of siRNAs (small interfering RNAs) and miRNAs (microRNAs) (see Chapters 10 and 12) provided powerful tools for controlling the expression of individual genes.

Recall that siRNAs and miRNAs are small RNA molecules that combine with proteins to form a RNA-induced silencing complex (RISC). In a process called RNA interference, or RNAi, the RISC pairs with complementary sequences on mRNA and either cleaves the mRNA or prevents it from being translated. Molecular geneticists have exploited this natural machinery for turning off the expression of specific genes. Silencing a gene with siRNAs or miRNAs and observing the effect can often be a source of insight into the gene’s function.