Breast cancers and most malignant tumors are composed of heterogeneous tumor cells both at genetic and morphological levels; intra-tumor heterogeneity can be one underlying cause of therapeutic resistance. Classical studies have focused on analyses of the relationship between primary tumors and metastatic dissemination, and on subclone evolution. However, it should be noted that tumor heterogeneity at the level of protein expression (proteomics) has not been yet studied in depth. The differences in protein expression also can play an important role in elucidating the relationship between intra-tumor heterogeneity and resistance to systemic therapy. In fact, in human tumors there is not always a homogeneous expression of many of the crucial factors involved in cell signaling, such as pMAPK, pAKt, pMTOR, even with constitutive oncogenic alterations upstream, such as HER2, PI3 K. Conversely, two of these factors, peIF4E and p4E-BP1, which are downstream, and control protein translation, show a diffuse and strong protein expression. In summary, most of cell signaling factors show a heterogeneous expression, regardless of oncogenic alterations. Tissue heterogeneity could be driven by local factors, including hypoxia. The fact that the phosphorylation of crucial proteins such as 4E-BP1 and eIF4E is observed homogeneously throughout most tumors and are druggable opens the chance to get real potential targets in cancer therapy.