"The evidence is extremely strong right now" that microRNAs play a fundamental role in cancer," says Slack, "and it's getting stronger and stronger every day."
Cancer is not the only disease in which microRNAs are emerging as important players. Studies now suggest that these miniature genes are involved in immune system function, heart disease, schizophrenia, Alzheimer's disease and Tourette's syndrome. Beyond that, there is a long list of diseases that appear to have a genetic basis, but for which no conventional gene has been identified. Thomas Gingeras, a genome researcher at Cold Spring Harbor Laboratory in New York, believes some of these diseases will ultimately be linked to microRNAs. "I think it's undoubtedly going to be the case," he says.
Perhaps that's because the tiny molecules exert so much influence over the rest of the body. Scientists estimate that humans have around 1,000 microRNA genes, which seem to control the activity of at least a quarter of our 25,000 protein-coding genes. "We are astounded by that number and believe it's a minimum," says Nobel laureate Phillip Sharp of M.I.T., in whose laboratory microRNAs are studied.
No wonder, then, that some scientists express embarrassment and regret that they failed to find microRNA genes sooner—chiefly because they didn't challenge basic assumptions about genes.
"It wasn't a technological issue," says Joshua Mendell, a microRNA researcher at Johns Hopkins. "The technology that's required to study microRNAs is not different from the technology used for the last couple of decades," he says. "It was more of an intellectual barrier."
Even Croce, for all his success, regrets that he didn't recognize microRNAs earlier. In the late 1980s, his team was pursuing a cancer gene in a stretch of DNA that did not code for any proteins. "So we trashed the project," says Croce. Now he knows that the gene was a microRNA. "Bias," he says, "is a bad, bad thing."
Sylvia Pagán Westphal is a writer living in Boston who specializes in covering genetics, biology and medicine.