Although known to researchers for decades, RNA had always been considered a mere servant to the more fundamental DNA. Though both kinds of nucleic acid are made of strings of nucleotides, the building blocks of the genetic code that determines every individual’s unique makeup, RNA generally has just one strand of code, while DNA has two. Encapsulated in the cell’s nucleus, DNA holds an organism’s entire archive of genes. To tap that archive, the organism creates RNA, a complementary string of nucleotides that is a copy of a section of DNA code. Exiting the nucleus, the RNA—in this capacity, called messenger RNA, or mRNA—enters the cytoplasm, where the code is translated into proteins.

By the 1990s, scientists had begun to suspect that RNA might play another important role. Craig Mello, at the University of Massachusetts, and Andrew Fire, then at the Carnegie Institution of Washington and now at Stanford University, were intrigued by studies of worms showing that injected RNA could sometimes interfere with the normal protein production coded by a particular gene. So they decided to inject two forms of RNA into Caenorhabditis elegans, a millimeter-long worm often used as a simple model of human disease. The first form was the better-known single-stranded RNA, while the second was a double-stranded cousin (dsRNA) found naturally only in viruses, in which the second strand contains the complementary code sequence of the first (both strands differ somewhat in structure from DNA).

Mello and Fire used this method to introduce extra copies of certain genes into the worm and then tested whether its behavior and appearance had changed. They hypothesized that the genes they had injected would be turned off. In fact, protein production associated with the genes carried by the double-stranded RNA was almost nil. The shutdown was powerfully specific, much more so than that elicited by the single-stranded RNA. It affected only those genes targeted, and it was easy to elicit. They called the effect RNA interference.

Mello and Fire described their worm experiments in the journal Nature, detailing research for which they were awarded the 2006 Nobel Prize in Physiology or Medicine. But it was only later that they and other researchers discovered how RNAi shuts down protein production. It turns out that double-stranded RNA attaches to a cell enzyme called Dicer, which chops the dsRNA (which the cell thinks came from an invading virus) into little pieces. A complex that contains the enzyme Argonaute 2 attaches to the dsRNA. Argonaute 2 splits those pieces into two single strands; one strand remains bound to the complex and eventually finds its corresponding messenger RNA. That mRNA, without this interference, would deliver the genetic code for the gene in question to the cytoplasm’s protein-making machinery, and the protein coded by the gene would be produced. Instead, Argonaute 2 cleaves the mRNA, rendering it useless. Even tiny amounts of dsRNA are enough to slam the door almost completely on protein production.

RNAi also works in fruit flies, plants, zebrafish and other lower organisms, but for several years that seemed to be as far as it went—no one could get RNAi to work in higher organisms. Double-stranded RNA injected into mammals appeared to turn off all genes. But everything finally changed in 2001 with the publication of a paper in Nature by Thomas Tuschl, a co-founder of Alnylam Pharmaceuticals in Cambridge, Mass. He knew that most RNAi experiments used long strands of dsRNA that strung together hundreds of nucleotides. But Tuschl and others had had success with shorter strands, especially in the fruit fly, so he decided to try that approach in mammalian cells.