Mammalian cells make only the single-stranded variety. If a cell’s defence mechanisms detect double-stranded RNA they destroy it, to protect against infection.

This aversity to double-stranded RNA means short hairpins can be used to knock out the messengers, thus nullifying the signal from the underlying gene.

In 1998, scientists identified RNAi, a mechanism that primitive organisms use to detect and destroy virus's double-stranded RNA and any viral mRNA.