Post Marketing Surveillance Study to Observe Safety and Efficacy of IBRANCE®

Inclusion Criteria * Women of ≥18 years of age with proven diagnosis of advanced/metastatic breast cancer (locoregionally recurrent or metastatic disease). * Estrogen Receptor+ (ER+) and/or Progesterone Receptor+ (PgR+) tumor based on local laboratory results (test as per local practice). * Human Epidermal Growth Factor Receptor 2 -(HER2-) breast cancer based on local laboratory results (test as per local practice or local guidelines) * Patients must be appropriate candidates for hormone therapy. * Evidence of a personally signed and dated informed consent document indicating that the patient (or a legally acceptable representative) has been informed of all pertinent aspects of the study. * During the post marketing surveillance (PMS) period, patients who initially administer IBRANCE® Exclusion Criteria * Known hypersensitivity to letrozole or its excipients or to any Cyclin-Dependent Kinase 4/6 (CDK4/6) inhibitor excipients. * Pregnant women