RESUMEN
Breast cancer is classified based on clinical stage, cellular morphology and immunohistochemical analysis. More precise prognostic factors are necessary to aid with therapeutic decisions. Breast cancer subtypes that differ in their genetic expression and prognosis have been determined using cDNA microarrays. These findings confirm the differences between the phenotypes and provide new knowledge about the biology of breast cancer. Based on the presence or absence of expression of the estrogen receptor (ER), breast cancer is divided in two groups: ER+ and ER-. Genetic expression profile has identified two subtypes of the ER+ tumors: luminal A and luminal B. ER- tumors also include two subtypes, the HER2+ and the basal type. These subtypes differ in their biology and both demonstrate short disease-free periods after treatment and poorer outcome. This classification has shown the relationship between cDNA microarrays and clinical outcome of these tumors. This classification is proposed as a method of identifying those patients who will demonstrate better results with the different adjuvant modalities.