Patient description: The patient is a 42 year-old postmenopausal woman who had a screening sonogram which revealed an abnormality in the right breast. She had no palpable masses on breast exam. Core biopsy was done and revealed a 1.8 cm infiltrating ductal breast carcinoma in the left upper outer quadrant. Lumpectomy was done and the surgical margins were clear. The tumor was HER2-positive and ER/PR negative. Axillary sampling revealed 1 positive lymph node out of 12 sampled. CXR was unremarkable. She is using “well women” multivitamins daily and no other medication. She smokes frequently and consumes alcohol occasionally. She is in a relation with only one partner and has a history of 3 pregnancies and live births. She breastfed all three children.

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  Breast Cancer Patients must have Stage I to III histologically confirmed invasive carcinoma of the breast. A minimum tumor size of at least 1.5 cm determined by physical exam or imaging (whichever is larger) is required Patients must have histologically confirmed hormone receptor positive (ER and/or PR), HER2 negative, invasive breast cancer. ER, PR and HER2 measurements should be performed according to institutional guidelines, in a CLIA-approved setting in the US or certified laboratories for Non-US regions. Cut-off values for positive/negative staining should be in accordance with current ASCO/CAP (American Society of Clinical Oncology/College of American Pathologists) guidelines. Central confirmation is not required for ER, PR, or HER statuses Patients with equivocal HER2 in situ hybridization results according to current ASCO/CAP guidelines are allowed, as long as the clinician has determined that they should be treated as HER2 negative For the window phase: Patients must have histologically confirmed invasive lobular carcinoma or invasive ductal carcinoma. No central confirmation of histological subtype is necessary for enrollment For the treatment phase: Patients with any histological subtype are eligible Women 18 years of age. Men are not eligible ECOG performance status 0 or 1 Required laboratory values Absolute neutrophil count ≥ 1,500/mm3 Platelets ≥ 100,000/mm3  Concurrent therapy with other Investigational Products Prior therapy with any CDK inhibitor Patients with Stage IV breast cancer are not eligible. Baseline staging to document absence of metastatic disease is not required, however is recommended as determined by institutional practice (in patients where there may be a reasonable suspicion of advanced disease e.g., large tumors, clinically positive axillary lymph nodes, signs and symptoms). If performed, reports of these examinations must be available. Examination type for staging, i.e. X-ray, sonography, bone scans, CT, MRI, and/or PET-CT, is at the discretion of the investigator History of allergic reactions attributed to compounds of chemical or biologic composition similar to palbociclib Patients receiving any medications or substances that are potent inhibitors or inducers of CYP3A isoenzymes within 7 days of randomization Uncontrolled illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, diabetes, or psychiatric illness/social situations that would limit compliance with study requirements. Ability to comply with study requirements is to be assessed by each investigator at the time of screening for study participation Pregnant women, or women of childbearing potential without a negative pregnancy test (serum or urine) within 7 days prior to randomization, irrespective of the method of contraception used, are excluded from this study because the effect of palbociclib on a developing fetus is unknown. Breastfeeding must be discontinued prior to study entry Patients with a history of any malignancy are ineligible except for the following circumstances Patients with a malignancy history other than invasive breast cancer are eligible if they have no active malignancy and are deemed by the investigator to be at low risk for recurrence of that malignancy Patients with the following cancers are eligible: ductal carcinoma in situ of the breast, cervical cancer in situ, and non-metastatic non-melanomatous skin cancers