Effect of Genotype-Guided Oral P2Y12 Inhibitor Selection vs Conventional Clopidogrel Therapy in Symptomatic ICAD

Inclusion Criteria: * Age ≥ 40 years old, male and female. * TIA or ischemic stroke secondary to symptomatic atherosclerotic stenosis of 30- 99% involving the intracranial ICA or MCA or posterior circulation arteries as evidenced by CT or MR angiography. * Index TIA or ischemic stroke event occurred within past 30 days. * Clinical indication for DAPT for at least 3 months. Exclusion Criteria: * Any contraindication to DAPT. * Any contraindication to use of clopidogrel (Plavix) or ticagrelor (Brilinta), such as pregnancy. A pregnancy test will be performed on all women of child-bearing age prior to enrollment in the study. * Indication for chronic anticoagulation based on guideline recommendations or investigator's judgment (e.g., atrial fibrillation, mechanical heart valve, intracardiac clot, dilated cardiomyopathy, ejection fraction \<30%, etc.). * Intracranial arterial occlusion (i.e. 100% stenosis) responsible for the acute brain ischemia. * Intracranial arterial stenosis secondary to causes other than atherosclerosis. * Extracranial carotid disease with a plan for carotid revascularization. * Intraluminal thrombus. * Unstable subdural hematoma within 12 months of randomization not amenable to embolization. * Previous spontaneous hemorrhagic stroke. * Traumatic brain hemorrhage within 1 month of randomization. * Living in a nursing home or requiring daily nursing care or assistance with activities of daily living. * Intracranial tumor (except meningioma) or any intracranial vascular malformation. * Life expectancy less than 6 months. * Enrolment in another study that would conflict with the current study.