A single DNA variant in the human genome can identify who will respond to hepatitis C virus (HCV) treatment and clear the virus. HCV is notoriously difficult to treat (Nat. Biotechnol. 27, 305–306, 2009). Individuals with HCV infection are given weekly injections of polyethylene glycol–conjugated interferon α-2a—Pegasys, from Roche of Basel, or Peg-Intron, from Schering-Plough in Kenilworth, New Jersey—plus daily oral ribavirin. But the 48-week-long therapy is poorly tolerated, and not always successful: only 40–50% of infected individuals clear the virus. Now a study by Dongliang Ge at Duke University in Durham, North Carolina suggests it may be possible to tailor antiviral courses to the most appropriate patients. The Duke researchers analyzed 1,137 infected individuals from multi-ethnic backgrounds and their response to treatment in a genome-wide association study (Nature 461, 399–401, 2009). They identified a group of single-nucleotide polymorphisms (SNPs) associated with treatment response in the region of the IL28B gene, which encodes an interferon-λ involved in viral suppression. About 80% of those individuals carrying two copies of an advantageous SNP cleared the virus during treatment. “It's a striking finding,” says Raymond Chung, a hepatologist at Boston's Massachusetts General Hospital. Chung believes additional prospective studies could turn this SNP into a “useful clinical tool” if combined with other variables. Schering-Plough, who owns rights to Ge's SNP, is looking to develop a genetic test based on this marker.