RNA inactivation (RNAi) is a relatively newly-discovered mechanism in which the presence of small fragments of dsRNA homologous to a given gene suppresses the expression of that gene.
It is a highly potent and specific process which is actively carried out by special mechanisms in the cell. It is thought to have evolved as a defence against the expression of retrovirus genes integrated into the host genome as provirus.
It was first discovered in plants, but has recently found to be a widespread effect in many eukaryotes.
It has recently been applied as an experimental technique to "knock out" genes in model organisms for experimental analysis. It may have medical applications.