Patients with mesenchymal/MpBC treated with temsirolimus-based regimens were evaluated...Of the eight patients who achieved SD ≥6 months/CR/PR, all 4 patients with available tissue had a molecular aberration that activate the PIK3CA/Akt/mTOR axis: PIK3CA mutation = 2; PTEN loss = 1; NF2 aberration = 1.