Inclusion Criteria:

          -  Adult women with proven diagnosis of advanced adenocarcinoma of the breast
             (locoregional recurrent or metastatic disease).

          -  Women who are not of childbearing potential.

          -  ER-positive and/or Progesterone receptor (PgR)-positive tumor based on local
             laboratory results (test as per local practice).

          -  HER2-negative breast cancer based on local laboratory results (test as per local
             practice or local guidelines).

          -  Patients must be appropriate candidates for letrozole therapy.

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0-2.

          -  Adequate bone marrow function.

          -  Adequate liver function

          -  Adequate renal function.

        Exclusion Criteria:

          -  Known hypersensitivity to letrozole, or any of its excipients, or to any palbociclib
             excipients.

          -  Current use of food or drugs known to be potent inhibitors or inducers of CYP3A4
             isoenzymes within 7 days prior to study entry.

          -  Prior treatment with any CDK inhibitor.

          -  Previous participation in a palbociclib clinical study.

          -  Participation in other studies involving investigational drug(s) within 2 weeks prior
             to study entry and/or during study participation.

          -  QTc >480 msec; history of QT syndrome, Brugada syndrome or known history of QTc
             prolongation, or Torsade de Pointes.

          -  High cardiovascular risk, including, but not limited to recent myocardial infarction,
             severe/unstable angina and severe cardiac dysrhythmias in the past 6 months prior to
             enrollment.

          -  Diagnosis of any second invasive malignancy within the last 3 years prior to
             enrollment. Note: patients with adequately treated basal cell or squamous cell skin
             cancer, a history of intraepithelial neoplasia or in situ disease (eg, carcinoma in
             situ of the cervix or melanoma in situ) may enter.

          -  Active uncontrolled or symptomatic brain metastases. Previously treated and clinically
             stable, brain metastases are permitted.

          -  Other severe acute or chronic medical or psychiatric conditions.

          -  Patients who are investigational site staff members directly involved in the conduct
             of the study and their family members, site staff members otherwise supervised by the
             investigator, or patients who are Pfizer employees directly involved in the conduct of
             the study.