Breast cancer is a multifactorial heterogeneous disease, reflected in a wide range of phenotypic subsets of tumors with varied degrees of aggressiveness and a significant global impact on women’s health. In addition to defining the profiles of breast tumors, it is necessary to identify the individual gene and protein expression aberrations and their impact on the biology of the tumor. Molecular pathology changed the way we think about the classification of breast cancer, by no longer relying on just the histological alterations, but also on their biologic pathways. However, it should be noted that although the identification of breast cancer genes contributes to the detection of precursor lesions and prevention of invasive disease, a correlation between phenotype and genotype is necessary, as the sole assessment of gene alterations is insufficient for the identification of predictive and prognostic factors allowing the application of new and individualized cancer therapies. In conclusion, this chapter focuses on the basic molecular pathology knowledge needed in everyday routine practice.
