The PALOMA-2 [A Study of Palbociclib (PD-0332991) + Letrozole vs. Letrozole For 1st Line Treatment Of Postmenopausal Women With ER + /HER2- Advanced Breast Cancer] and PALOMA-3 [Palbociclib (PD-0332991) Combined With Fulvestrant In Hormone Receptor + HER2-Negative Metastatic Breast Cancer After Endocrine Failure] clinical trials have proven that concomitant use of palbociclib to standard endocrine therapy significantly improved outcomes in treatment of ER-positive, HER2-negative advanced breast cancer [20, 21]. The gene discussed is ESR1; the disease is breast cancer.