 The objective of the present study which is entitled Peptide Profiles of Women Undergoing Breast Biopsy is to determine whether the diagnosis of malignancy on breast biopsy is associated with a specific serum or plasma peptide profile which can be distinguished from the peptide profile associated with benign diagnoses on breast biopsy. To accomplish this, blood samples will be collected from 500 women undergoing breast biopsy and 100 women with metastatic disease. Samples will be obtained prior to the biopsies and analyzed by mass spectrometry. Residual material will be shared with investigators of the National Cancer Institute Clinical Proteomics Technology Assessment Consortium. 