Short course of letrozole before surgery may reveal breast Cancer's hormone dependence

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: 1. Eligibility waivers are not permitted. Subjects must meet all of the inclusion and exclusion criteria to be registered to the study. Study treatment may not begin until a subject is registered. 2. Patients must provide informed written consent 3. ECOG performance status 0-2. 4. Clinical stage operable I, II or III invasive mammary carcinoma, which is ER-positive by IHC and HER2-negative by Herceptest (0 or 1+) or not amplified by in situ hybridization as per routine clinical testing. * Patients who have measurable residual tumor at the primary site * Patients who will undergo surgical treatment with either segmental resection or total mastectomy 5. Measurable tumor. Measurable disease: a mass that can be reproducibly measured by physical exam, mammogram or ultrasound and is at least 1 cm in size 6. Post-menopausal female subjects ≥18 years of age, as defined by any of the following: * Subjects at least 55 years of age; * Subjects under 55 years of age and amenorrhoeic for at least 12 months or follicle-stimulating hormone (FSH) values ≥40 IU/L and estradiol levels ≤40 pg/mL (140 pmol/L) or in postmenopausal ranges per local or institutional reference ranges; * Prior bilateral oophorectomy or prior radiation castration with amenorrhea for at least 6 months. * (There is no upper age limit for enrollment to this study) 7. No prior chemotherapy for this primary breast cancer. 8. Patients with a prior history of contralateral breast cancer are eligible if they have no evidence of recurrence of their initial primary breast cancer. 9. Women may have been taking tamoxifen or raloxifene as a preventive agent prior to study entry but must have discontinued the drug for at least 14 days prior to study enrollment. 10. Subjects must have ended hormone replacement therapy (HRT) (e.g., conjugated estrogens tablets, USP, \[Premarin\]), at least 7 days prior to receiving the first dose of randomized therapy. 11. Patients must have adequate hepatic and renal function. All tests must be obtained less than 4 weeks from study entry. This includes: 1. Creatinine \<2X upper limits of normal 2. Bilirubin, SGOT, SGPT \<1.5X upper limits of normal 12. Able to swallow and retain oral medication Exclusion Criteria: 1. Patients with locally advanced disease who are candidates for other preoperative chemotherapy at the time of initial evaluation. This may include patients with locally advanced disease such as: * Inflammatory breast cancer (T4d) * Fixed axillary lymph node metastases (N2) * Metastasis to ipsilateral internal mammary node (N3) 2. Locally recurrent breast cancer 3. Evidence of distant metastatic disease (i.e. lung, liver, bone, brain, etc.) 4. Serious medical illness that in the judgment of the treating physician places the patient at high risk of operative mortality. 5. Severe uncontrolled malabsorption condition or disease (i.e. grade II/III diarrhea, severe malnutrition, short gut syndrome) 6. Dementia, altered mental status, or any psychiatric condition that would prohibit the understanding or rendering of informed consent. 7. Use of an investigational drug within 30 days or 5 half-lives, whichever is longer, preceding the first dose of letrozole.