The purpose of this study is to compare pre and post Aromatase Inhibitor (AI) treatment serological markers of inflammation and assess for correlation with AI-MS.

Inclusion Criteria:

• Women 18 years and older with non-metastatic hormone receptor (ER and/or PR) positive breast cancer who are eligible for and planning on starting treatment with an AI within one month of signing consent.
• Postmenopausal, defined as at least one of the following: a) amenorrhea for at least 12 months b) prior bilateral oophorectomy at least 2 years prior to trial registration.

Exclusion Criteria:

• History of autoimmune connective tissue disease, such as rheumatoid arthritis, lupus, or other autoimmune conditions affecting joints.
• Treatment with steroid (for any condition, except for chemotherapy premedication) within 30 days of trial registration.
• Prior treatment with an AI (patients previously or currently on tamoxifen are eligible as long as patients are off tamoxifen for 2 weeks prior to baseline blood draw)
• Active or ongoing infection
• Known metastatic disease
• Known history of HIV or hepatitis infections
• Ongoing radiation therapy (radiation must have been completed 2 weeks prior to starting treatment with an AI)
• Non-invasive (DCIS, LCIS) cancer only (no diagnosis of invasive cancer)
• Pregnant or lactating
• Unable to speak, read, and write in English

Women 18 years and older with non-metastatic hormone receptor (ER and/or PR) positive breast cancer who are eligible for and planning on starting treatment with an AI within one month of signing consent