200 patients undergoing percutaneous coronary intervention (PCI) for treatment of non-ST-elevation acute coronary syndrome or stable coronary artery disease will be eligible for enrollment. Patients will be randomly assigned to a strategy of genotyping(using Taqman genotyping method) or standard treatment . CYP2C19*2 carriers will be given 90 mg ticagrelor twice daily, and non-carriers and patients in the standard treatment group will be given 75 mg clopidogrel daily. At the end of the 5 day antiplatelet treatment, efficacy of the treatment strategies will be evaluated using light transmittance aggregometry (LTA) method.