PIK3CA and breast cancer: Although activation of the PI3K signaling is associated with the poor-prognosis luminal B subtype, and is accompanied by the development of endocrine therapy resistance, and although inhibition of the PI3K pathway signaling in endocrine-resistant breast cancer improves the response to hormone therapy [16], the acquisition of PIK3CA mutations did not appear to be responsible for the development of endocrine resistance, per se [17].