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1. In a sick cell, the nucleus dispatches single-stranded messenger RNA [blue] to deliver the genetic blueprints for disease-causing proteins. 2. Double-stranded RNA–injected into the patient’s eye–migrates into the cell. 3. Once inside, it joins with a cluster of proteins that unravels the double strand. 4. One RNA strand, still attached to the protein complex, seeks out the diseased messenger RNA and slices it in two. 5. An enzyme cleans up the cleaved messenger RNA, thus preventing it from creating the proteins that cause disease.

In the seven years since scientists discovered RNA interference, or RNAi–a way of hijacking a cell’s defense mechanism to silence
defective genes–the technology has been hailed
as a potential treatment for everything from cancer
to coronary heart disease. Now scientists are putting it to the test in humans.

This past November, Acuity Pharmaceuticals in Philadelphia began testing an RNAi-based therapy in 12 patients suffering from age-related macular degeneration, the leading cause of blindness in people over 55. The treatment, which involves an injection of engineered RNA strands into the patient’s eyes, is a simple, if not painless, process [see illustration]. The strands are designed to intercept faulty instructions sent by sick genes inside eye cells, thereby halting the deterioration of the macula. Although
scientists are still unsure about potential side effects, given the alternative of almost certain blindness, patients feel that it’s worth the risk.