PIK3CD and breast cancer: In breast cancer, a gene expression signature predictive of in vitro sensitivity to the PI3K inhibitor GDC-0941 (O’Brien et al., 2010), and a PIK3CA mutation associated gene signature (PIK3CA-GS) derived from exon 20 PIK3CA mutations able to predict PIK3CA mutation status in primary breast cancers and predictive of Rapamycin response in vitro have recently been described (Loi et al., 2010).