This study will be an open-label prospective observational trial designed to test associations between polymorphisms of candidate genes and tamoxifen. Pre- and post-menopausal women taking tamoxifen as standard therapy or chemopreventive therapy will be included in this study.

This study will be an open-label prospective observational trial designed to test associations between polymorphisms of candidate genes (focusing initially on the UGT2B7 enzyme) and tamoxifen (TAM) toxicity. Pre- & post-menopausal women (aged ≥18 years) taking TAM (20 mg/day) as standard therapy or chemopreventive therapy will be included in this study. Patients will be enrolled after they complete all primary surgery, radiation, and adjuvant chemotherapy. Patients will be excluded if they are undergoing other adjuvant endocrine therapies. Other reasons for exclusion will include patients who are pregnant or lactating, or are currently on corticosteroids, phenobarbital, or megestrol acetate. The goal will be to recruit a total of 45 eligible patients over a 1 year accrual period. The investigators expect to treat ~50 new patients per year with TAM at the standard dose of 20 mg/day.

Inclusion Criteria:

• Patients receiving tamoxifen for adjuvant therapy of breast cancer or ductal carcinoma in situ, or as chemoprevention
• Age 18 years and above
• May be pre- and post-menopausal
• Females
• Patients may be at any point in their hormonal treatment, but must have completed any planned surgery, radiation and chemotherapy
• Must use a reliable form of birth control

Exclusion Criteria:

• Pregnant
• Breastfeeding
• Concurrent use of corticosteroids, megestrol, or phenobarbital
• History of allergy to tamoxifen
• Unwilling to have a yearly gynecological exam

Females, age 18 years of age and above. Patients receiving tamoxifen for adjuvant therapy of breast cancer or ductal carcinoma in situ or as chemoprevention.

After determining if data is valuable