ESR1 and breast cancer: In particular, palbociclib (PD-0332991 or Ibrance) has shown clear benefits in Phase II (PALOMA-1) and III (PALOMA-2 and -3) clinical trials in advanced estrogen receptor-positive (ER+) breast cancers, doubling the progression-free survival (PFS) compared to letrozole or fulvestrant alone2, 3, 4.