Since last April, 19 cancer patients whose liver tumors hadn’t responded to chemotherapy have taken an experimental drug. Within weeks of the first dose, it appeared to work, by preventing tumors from making proteins they need to survive. The results are preliminary yet encouraging. With a slight redesign, the drug might work for hundreds of diseases, fulfilling the promise that wonder cures like stem cells and gene therapy have failed to deliver.

The biotech company Alnylam announced in June that its drug ALN-VSP cut off blood flow to 62 percent of liver-cancer tumors in those 19 patients, by triggering a rarely used defense mechanism in the body to silence cancerous genes. Whereas conventional drugs stop disease-causing proteins, ALN-VSP uses RNA interference (RNAi) therapy to stop cells from making proteins in the first place, a tactic that could work for just about any disease. “Imagine that your kitchen floods,” says biochemist and Alnylam CEO John Maraganore. “Today’s medicines mop it up. RNAi technology turns off the faucet.”

Here’s another analogy: If DNA is the blueprint for proteins, RNA is the contractor. It makes single-stranded copies of DNA’s genes, called mRNA, which tell the cell to produce proteins. In 1998, scientists identified RNAi, a mechanism that primitive organisms use to detect and destroy virus’s double-stranded RNA and any viral mRNA. Mammals’ immune systems made RNAi’s antiviral function irrelevant (although all vertebrates, including humans, still use RNAi to regulate mRNA activity), but researchers found that introducing small segments of double-stranded RNA to cells could trigger the ancient mechanism and selectively halt the production of specific proteins.

That ability makes RNAi a potential fix for many diseases, including cancer, that arise when abnormal cells produce excessive amounts of everyday proteins. In theory, manipulating RNAi to kill proteins is simple. ALN-VSP, for example, consists of synthetic double-stranded RNA designed to match tumor mRNA that codes for two proteins: VEGF, which cancers overproduce to help grow new blood vessels, and KSP, which sets off rapid cell division. The researchers send the synthetic RNA into liver cells, and the body’s RNAi system kills both the synthetic RNA and any matching tumor-grown mRNA. Knock out the mRNAs coding for those proteins—which in the liver are produced only by cancer cells—and the tumor stops growing.