Among patients with minor ischemic stroke or transient ischemic attack, the use of clopidogrel plus aspirin compared with aspirin alone reduced the risk of a new stroke only in the subgroup of patients who were not carriers of the CYP2C19 LOF alleles in a CYP2C19 genetic analysis of the CHANCE trial (23). The gene discussed is CYP2C19; the disease is Stroke.