The treatment currently recommended for chronic infection with the hepatitis C virus (HCV) involves a course of peg-interferon alpha combined with ribavirin of variable duration. In the face of the elevated costs of the two drugs and the potentially serious adverse events related to combination therapy, the identification of baseline predictors of treatment response that may identify patients with little or no chance of achieving a sustained virological response is of paramount importance. In 2009, a genome-wide association study on the IDEAL (Individualized Dosing Efficacy vs. flat dosing to Assess optimal, pegyLated interferon therapy) study population and a small group of patients from another cohort identified rs12979860, a single nucleotide polymorphism (SNP) located on chromosome 19 near the IL28B gene encoding for interferon lambda-3, was found to be strongly associated with sustained virological response (SVR). The independent external validation of this pivotal study, coupled with the demonstration that the IL28B SNP affects treatment outcome in other HCV genotypes has led to the incorporation of this test into the current pretreatment algorithm of HCV-infected patients.