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Glycoprotein (GP) Ia/IIa is a major platelet-collagen receptor playing a key role in thrombosis following collagen exposure. The 807 C/T polymorphism of the GP Ia gene (ITGA2) has been associated with platelet GP Ia/IIa receptor expression, having T-allele carriers, increased receptor density and thrombotic risk. The aim of the study was to assess the role of the 807 C/T polymorphism on modulating platelet function in patients undergoing coronary stenting receiving a 300 mg clopidogrel loading dose. Platelet aggregation was assessed in 44 patients by light transmittance aggregometry following adenosine diphosphate and collagen stimuli at baseline, and 10 min, 4 h and 24 h after clopidogrel front loading. The T allele was found in 73% of patients. Clopidogrel reduced adenosine diphosphate-induced platelet aggregation (P< 0.01), which was similar in carriers and non-carriers of the T allele throughout the study (P = 0.73). Clopidogrel reduced collagen-induced platelet aggregation only in non-carriers of the T allele (P = 0.03), which resulted in an increase in T allele carriers during the overall study (P = 0.04). In conclusion, the T allele of the GP Ia gene modulates platelet aggregation and clopidogrel antiplatelet effects, suggesting an enhanced reactivity to fibrillar collagens (exposed during coronary stenting) in T allele carriers and might contribute to an increased thrombotic risk in these patients.