The purpose of this study is to identify possible genetic polymorphisms that contribute to specific toxicities associated with capecitabine (hand-foot syndrome, diarrhea, and neutropenia).

Additionally, this study will look at gene polymorphisms in patients experiencing the toxicities of interest, the frequency of polymorphisms and differences in drug metabolism.

Inclusion Criteria:

• women with breast cancer in whom single agent capecitabine therapy is being considered
• aged 18 years and older

Exclusion Criteria:

• patients who have previously received capecitabine are excluded
• patients cannot be receiving capecitabine in combination with another cancer chemotherapy; concurrent use of trastuzumab is not permitted; concurrent use of zoledronic acid is allowed
• serum albumin less than 3.0 g/dL within the last 30 days
• creatinine clearance (CrCL) or glomerular filtration rate (GFR) less than 60 mL/min [/body surface area (BSA)] (within the last 30 days)
• inability to understand and give informed consent to participate
• patients with a history of inflammatory bowel disease requiring therapy or patients with chronic diarrhea syndromes or paralytic ileus
• patients with prior or concurrent pelvic irradiation
• patients who use an ostomy for fecal excretion
• there is no limit on the number of prior chemotherapies; the decision to use capecitabine is determined solely by the treating physician

Patients receiving treatment with capecitabine for breast cancer at a participating academic medical center.