Purpose: Mutations that activate the PIK3CA oncogene and inhibit the tumor suppressor gene PTEN action are commonly found in breast tumors. Aid is a key activator of cell survival. p53 is frequently found mutated in human tumors, and mutant p53 protein actively contributes to tumorigenesis. In selected cases of breast cancer, trastuzumab (TZMB) is incorporated in the primary treatment in the adjuvant and metastatic settings. Many studies have reported that selected patients are resistant to TZMB due to the presence of p95 HER2 fragments. To address this, we analyzed PTEN, Akt, MAPK, p53 and p95 expression in breast cancer patients treated with TZMB.