New developments in messenger RNA and RNA interference promise to speed the discovery of new treatments for diseases and genetic defects.

• The ability to target virtually any protein: A key limitation of small-molecule and protein drugs is that they can only target certain classes of protein. Most of the targets for currently marketed small-molecule drugs fall into one of a small number of protein classes: G-protein-coupled receptors, ion channels, enzymes and nuclear hormone receptors. Despite much effort, attempts to find small-molecule drugs targeting other classes of protein have been largely unsuccessful. The range of targets for protein drugs is also limited, mainly to cell-surface receptors or to circulating proteins which bind to them. In contrast, given the availability of the base sequence of the entire human genome, it should be possible to design siRNAs specific for each and every gene/mRNA in the body.
  This opens up the possibility of developing siRNA drugs for the many different proteins that do not fit into the so-called "druggable target classes" for small-molecule and protein drugs but which appear to play important roles in diseases. A number of these "intractable" but well validated targets already exist, and are a focus for initial work on RNAi therapeutics.
  
• Simplified discovery of drug candidates: Identification of appropriate drug candidates can be more straightforward for siRNAs than it is for small-molecule or protein drugs. In contrast to the extensive lead optimization steps required in small molecule
  and protein drug discovery, RNAi drug candidates can be identified using bioinformatics tools to select sequences complementary to the target mRNA. The process of choosing an RNAi-based drug candidate may simply involve the synthesis and testing of a relatively small number of siRNAs, incorporating chemical modifications to confer stability and direct the siRNA to the appropriate tissues and cells, and/or mixing the siRNA with appropriate delivery agents to achieve the same goals.
  

See Alnylam for more details.