ESR1 and neoplasm: These results are consistent with the results of phase Ib clinical trial NCT01791478 showing that a small number of patients (~10%) with KRAS mutations do not derive clinical benefit by alpelisib, an α-selective PI3K inhibitor, in combination with letrozole in ER+/HER2- metastatic breast cancer [62], and of Poseidon phase 1b trial [45], in which mutations of KRAS in circulating tumor (ct) DNA identified patients with clinical resistance to taselisib [63].