Breast cancer is a heterogeneous disease, with various subtypes demonstrating differing biological behavior, prognosis, and response to therapeutic agents. Improving the understanding of the biology of breast cancer can result in better prognostication and therapy in individual patients, and has important clinical implications. While valuable scientific knowledge is generated from studies of breast cancer cell lines or animal models, such information generated in vitro may not fully reflect the in vivo model. Direct analysis of patient samples is important to validate in vitro findings, and represents a step closer to clinical application. Studying patient tumor samples for genetic and protein markers and correlating these analyses with clinical characteristics and outcomes could provide valuable insights into tumor biology in vivo, and leads to better understanding of tumor biology and resistance mechanisms. This is a single-centre study of biomarker analysis in breast tumors. A total of 400 patients with breast lesions for whom a diagnostic core biopsy is planned will be enrolled over 2 years. During the diagnostic tumor core biopsy, 3-4 additional tumor samples will be obtained through the same needle track for the purpose of this study. The tumor cores will be stored in liquid nitrogen for subsequent histopathological analysis; DNA, RNA and protein will be extracted from tumor cores for genetic, gene expression and proteomics studies. The patient's clinical progress will be tracked through the clinical case files. For patients in whom the clinical diagnostic biopsy yields a benign result, the samples obtained from them will serve as control specimens. For patients in whom the clinical diagnostic core biopsy confirms the diagnosis of breast cancer, the samples will be considered cases, and the following information will be tracked: histopathological characteristics of the tumor including ER/PR/c-erbB2 status and other known prognostic and predictive immunohistochemical markers (e.g., Ki67, p53, etc), anti-cancer treatment, progression-free and overall survival.