ESR1 and breast carcinoma: The 1L indication for palbociclib plus an AI is supported by results from the phase 3 PALOMA-2 trial (10), which demonstrated significant improvement in progression-free survival (PFS) in patients with estrogen receptor-positive/HER2− advanced breast cancer receiving palbociclib plus letrozole compared with those receiving placebo plus letrozole (27.6 vs. 14.5 months; hazard ratio [HR], 0.563 [95% confidence interval (CI), 0.461−0.687]; P < 0.0001) (12).