 The proposed study would retrospectively test CYP2D6 genotypes in 200 - 300 estrogen receptor ER positive breast cancer patients who are treated with tamoxifen post-operatively. Blood collected with informed consent would be used for examining the patients CYP2D6 genotype, and identifying those who are CYP2D6 poor metabolizers CYP2D644 genotype, and for measurement of endoxifen blood level in those women who are on the drug. 