The RNA interference (RNAi) gene-silencing mechanism is induced by double-stranded RNA (dsRNA) and is highly sequence-specific. It is an extremely powerful tool for silencing gene expression in vitro, and might be used as therapy in human pathologies such as cancer, viral infections and genetic disorders. RNAi was initially discovered in plants, but it has become clear that it is also conserved in animal species. Triggering of RNAi by the introduction of small dsRNA (or small interfering RNA) into living cells as a tool to inhibit the expression of specific genes holds the promise to selectively extinguish the expression of disease-associated genes in humans. On the other hand, RNAi technology will serve to elucidate the functions and interactions of the thousands of human genes in high-throughput systems and help in target validation technology.