PIK3CA and breast cancer: However, recent studies have shown that PTEN-deficient breast cancer cell lines, which are resistant to PI3K inhibitors, have a highly activated p110β D1067Y mutant and induce elevated phosphatidylinositol 3,4,5-trisphosphate (PIP3) levels, resulting in hyperactivation of the PI3K pathway (Nakanishi et al. 2016).