Twenty-five years ago no one had even heard of the hepatitis C virus. Today it is a leading cause of liver cancer and a major reason why people get liver transplants. Globally it kills about 350,000 people a year; in the U.S., more people now die of hepatitis C than of AIDS.

The infection can be cured—albeit with debilitating side effects. Standard treatment with interferon and ribavirin causes fever, headaches, fatigue, depression and anemia. Such therapy may last as long as 11 months and clears the infection in 50 to 70 percent of cases. The recent addition of protease inhibitors, a class of medications that was first used against HIV, has improved cure rates and lessened treatment time. Unfortunately, the newer drugs work only against the type of hepatitis C most common in North America, Europe and Japan, so they are not equally effective around the world.

RNA medications may better that outlook. In 2013 researchers showed that targeting a particular microRNA in liver cells with an experimental drug called miravirsen dramatically decreased the amount of hepatitis C virus in most patients receiving treatment, in some cases to undetectable levels. The experimental medication consists of a short sequence of DNA whose “letters” are exactly complementary to the RNA letters found on the microRNA, allowing the drug to home in on its objective precisely.

The microRNA in question, known as miR-122, plays a key role in the production of many proteins in the liver. It seems to enhance their manufacture, however, rather than suppressing it as so many microRNAs do. Once the hepatitis C virus gains entry to a cell, it attaches itself to miR-122, ensuring that multiple viral copies are made. Blocking miR-122 ends up blocking virus replication as well.

The main side effect of miravirsen therapy was redness at the injection site, which eventually disappeared. Because the treatment aims at something in the host cells—as opposed to one of the viral proteins (which is how the protease inhibitors work)—it should be effective against all strains of hepatitis C.

Although the intervention was designed to last just four weeks (the infection eventually returned in all the treated patients), there is reason to believe that longer treatment with miravirsen will prove more effective. “The thought is that if you block the viral replication long enough, you can cure the disease,” says Harry L. A. Janssen, a senior scientist at the Toronto General Research Institute and a co-author of the miravirsen study, which was published in the New England Journal of Medicine. Further tests are ongoing.