Carriers of CYP2C19 LoF alleles are at a significantly increased risk of secondary vascular events following percutaneous coronary intervention, and ischemic stroke or TIAs.6, 7, 8, 9, 10, 11 In these patient groups, genotype-guided prescribing has been shown to be a cost-effective strategy, and some health systems have introduced CYP2C19 genotyping into routine practice.12, 13, 14 If adopted routinely, the most recent recommendations from NICE would result in approximately 100,000 additional genetic tests each year in the United Kingdom given the incidence of ischemic stroke and TIA.15 The gene discussed is CYP2C19; the disease is ischemic stroke.