PIK3CA and breast cancer: In this study suppression of Ki-67 proliferating index, which is a validated intermediate endpoint biomarker for endocrine sensitivity was used as a surrogate marker of response.31 Furthermore, another study showed that patients with PIK3CA mutated early-stage breast cancer either treated with tamoxifen or untreated had improved outcome in comparison to PIK3CA non-mutated patients.32 These data were concordant with previous findings from in vitro study showing that PIK3CA mutated cell lines were more sensitive to tamoxifen compared to PIK3CA wild type cell lines.33