The investigators hypothesize that the TCF20 rs932376 locus controls endoxifen levels in estrogen receptor (ER)-positive breast cancer patients on tamoxifen therapy and that the requirement of tamoxifen dose to achieve therapeutic levels of endoxifen varies according to the genotype status of the patient's at the TCF20 rs932376 locus.

Patients will receive escalating doses of tamoxifen according to their TCF20 rs932376 genotype status. Patients on 20mg tamoxifen daily for 8 weeks or longer will provide blood samples for analysis of tamoxifen and its metabolites. Patients assess with >30nM endoxifen concentrations will continue with 20mg tamoxifen dose. Patients who have endoxifen concentrations <30nM will receive dose increments of 10mg every 8 weeks until they achieve endoxifen threshold of >30nM or reach maximum dose level of 40mg/day.

Inclusion Criteria:

• Histologically or cytologically confirmed stage I to IV breast cancer
• Age greater than 21 years
• Estrogen- or progesterone-receptor positive tumours
• Patients receiving tamoxifen 20mg daily as monotherapy
• Patients currently taking tamoxifen in either adjuvant or metastatic setting for greater than or equal to 8 weeks (for Dose determination and confirmation phases)
• Patients who will commence on tamoxifen therapy in either adjuvant or metastatic setting (for Dose confirmation phase only)
• Written and informed consent from participating patients

Exclusion Criteria:

• Patients with expected survival less than 6 months
• Patients who are human epidermal growth factor receptor 2 (HER2)-postitive
• Inability to provide informed consent
• Patients receiving CYP2D6 inhibitors or inducers within the past four weeks from the time of study enrollment. However, patients started in CYP2D6 inhibitors or inducers after commencing on tamoxifen therapy will not be excluded from the study.
• Pregnancy
• Patients with prior malignancies other than those who have received curative treatment for basal cell carcinoma of the skin or carcinoma in situ of the uterine cervix
• Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/ social situations that would limit compliance with study requirements.
• Patients who have documented prior allergic reaction to tamoxifen