Doxorubicin is a topoisomerase-targeting anthracycline that is one of the most effective anticancer drugs currently known. However, its clinical use is restricted by cardiotoxicity and the development of drug resistance. The main goal of this thesis has been to increase the knowledge of doxorubicin mechanism in addition to evaluate if predictive biomarkers for doxorubicin response could be identified. A total of 128 tumor samples collected from breast cancer patients before and after neoadjuvant treatment with doxorubicin were studied. mRNA expression level in tumor tissue was assessed using whole-genome mRNA microarray analysis (Agilent Human GE 4x44K microarray).More than 5000 genes were found to be up- and down regulated following doxorubicin treatment. The molecular and cellular functions as well as canonical pathways found to be enriched in the list of genes up regulated after doxorubicin exposure were involved in among other cardiovascular system development and function, cellular movement and immune responses. p53 was found to be the transcription factor regulating the highest number of target molecules within the list of up regulated genes. RNA processing, splicing and translation were shown to be overrepresented in the list of down regulated genes. The association between doxorubicin response and changes in gene expression revealed several genes such as CTGF, ITGB4 and IGF1 to be up regulated in the samples collected from patients characterized with a partial response to doxorubicin compared to those with minimal change and/or stable disease following treatment. In addition, the gene expression profiles between samples having wild type compared to mutated TP53 were studied, and a lower induction of expression were found for several genes such as FGF9 and COL11A2 in the samples having a mutated p53. This study showed that the gene expression profile in breast cancer tumors is altered as a response to doxorubicin exposure. Identifying genes significantly altered after therapy and associate their change with response to treatment may help identify the subgroup of patients benefitting from doxorubicin treatment. Patients with little or no effect of treatment could receive alternative therapy and be spared unnecessary treatment and risk of side effects.