Disease-causing proteins can be inhibited by small-molecule drugs or by antibody drugs that bind to the protein, but there's a third class of drugs being developed that prevents the protein from being produced in the first place. They target mRNAs, the intermediary between genes (DNA) and the proteins they code for.

The nucleic-acid drugs bind to the mRNA and signal for destruction. Reduced levels of mRNA result in a reduction of the proteins they encode for.

RNA drugs generally come in two flavors. Isis Pharmaceuticals (NASDAQ:IONS) uses antisense to target the mRNA for destruction. The biotech has already proved that the technology works; its drug Kynamro, which is sold by partner Sanofi (NYSE:SNY), is already on the market. The biotech has a stocked pipeline of additional antisense drugs.

The other major class of RNA drugs is called RNAi, which use a different cellular mechanism to trigger degradation of the mRNA. The class isn't nearly as developed, because it was discovered later and companies had trouble targeting the drugs. Eventually they figured out how to target to the liver, so RNAi companies are going after liver diseases first.

Alnylam Pharmaceuticals' (NASDAQ:ALNY) lead program treats TTR-mediated amyloidosis, a rare genetic disease that causes misfolding of a protein in the liver that can get intertwined and deposited elsewhere, causing nerve and heart problems. Arrowhead Research (NASDAQ:ARWR) is developing a hepatitis B drug called ARC-520.

Watch the following video for further insight on Isis Pharmaceuticals, Alnylam, and Arrowhead Research from the Fool's senior biotech specialist, Brian Orelli, and health-care analyst David Williamson.

Brian Orelli and David Williamson have no position in any stocks mentioned. The Motley Fool recommends Alnylam Pharmaceuticals, Amazon.com, and Isis Pharmaceuticals and owns shares of Amazon.com. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.