RNA interference is a mechanism that inhibits gene expression at the translation stage or hinders gene transcription. It is described by three similar but phenotypically different phenomena: co-suppression in plants, quelling in fungi, and RNA interference in animals. Recent applications of RNA interference include modulating HIV replication, inhibiting HIV, treating hepatitis, intervening in atherosclerosis, and reducing heart and brain damage. RNA interference has diverse therapeutic applications including identifying gene functions, creating genome-wide screens, developing antiviral and anticancer therapies.