PI3K signaling is frequently altered in breast cancer (BC) as mutations of the PI3K catalytic alpha subunit (PIK3CA) are common events, occurring in 9–45% of BC according to the subtype, followed by PTEN loss of function (13–35%) and, less frequently, AKT substitutions (2–4%) or amplification (5–10%) (Cancer Genome Atlas Network 2012; Guerrero-Zotano et al., 2016; Brown and Banerji, 2017; Fruman et al., 2017). The gene discussed is PTEN; the disease is breast cancer.