Dose-determining and Dose-confirmatory Study to Investigate the Optimal Dose of Tamoxifen in Breast Cancer Patients According to Genotype Status of TCF20 rs932376

Patients taking 20mg tamoxifen will receive dose increments of 10mg every 8 weeks until endoxin threshold of \>30nM or the maximum dose of 40mg/day is reached. The escalated dose will be taken for 1 year before reverting back to 20mg.

Patients will take tamoxifen at the ascribed genotype-determined dose as determined in the dose-determination phase.