Drug resistance remains a major issue (Gonzalez-Angulo et al. 2007) and since the discovery that expression of the protein markers ER, PR and her-2/neu determines response to a given targeted therapy (Bast et al. 2001), the assessment of their expression in breast cancer has become an important first step in selecting a patient’s treatment (Bast et al. 2001). This evidence concerns the gene ESR1 and breast cancer.