PIK3CA and breast cancer: In particular, the integrins’ ligands collagens and fibronectin have been shown to be responsible for the decreased drug sensitivity of different BC cell lines to several treatments (i.e., paclitaxel, vincristine chemotherapy, tamoxifen, ionizing radiation, lapatinib, trastuzumab) through the activation of PI3K/AKT and Ras/Raf/MEK/ERK1-2 pathways (89).