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To assess the contribution of the rs3846662 polymorphism of HMGCR on serum lipid levels and statin efficacy, we measured in vivo HMGCR mRNA and lipid levels in French Canadian individuals affected by heterozygous familial hypercholesterolemia due to the deletion of more than 15 kb of the LDLR gene.Men and women carrying the AA genotype at rs3846662, and no APOE4 allele, had higher levels of total cholesterol (5.43 vs. 4.58 mmol/l, P0.05) and LDL-cholesterol (5.20 vs. 4.39 mmol/l, P0.05) at baseline. However, with regard to statin efficacy, the penetrance of the AA genotype was sex dependent. Indeed, the percentage reduction in LDL-cholesterol upon statin treatment was significantly decreased in women with the AA genotype compared with women without it (38.4 vs. 46.2%, P0.05), whereas this was not observed in men. Although both men and women bearing the AA genotype showed a higher ratio of full-length HMGCR mRNA/total HMGCR mRNA compared with individuals without it (n=37, P0.05), overall transcription of HMGCR was decreased and increased in men and women carrying this genotype, respectively (n=37, P0.01 and P0.05). Finally, in our familial hypercholesterolemia cohort, HMGCR alternative splicing explained between 22 and 55% of the variance in statin response.rs3846662 polymorphism and the alternative splicing of HMGCR mRNA significantly impact women's response to statin therapy.