Tumors are heterogeneous with varying response to chemotherapeutic agents. We hypothesize that tumors that are sensitive to a particular chemotherapeutic agent have a distinctive tumor protein profile compared to those that are resistant. We further hypothesize that since tumor is continuously perfused by serum, serum protein profile can be used as a surrogate marker of tumor protein profile. The primary objective of this study is to identify a serum protein profile that predicts gemcitabine/carboplatin sensitivity or resistance in breast cancer patients with prior exposure to anthracyclines and taxanes. Secondary objectives are to establish the serum protein profile of breast cancer patients who have had prior exposure to anthracyclines and taxanes, and to study the pharmacogenetics of gemcitabine toxicity by correlating germline genotype of transporters and drug metabolizing enzymes with plasma and intracellular gemcitabine pharmacokinetics.