ESR1 and aneurysmal bone cyst: In 2015, the combination of letrozole and palbociclib was approved for the treatment ER+/HER2− advanced breast cancer (ABC), while in 2016, the FDA approved the use of palbociclib in combination with fulvestrant for the treatment of ABC or metastatic breast cancer (MBC) HR+/HER2− in women who progressed after hormonal therapy [42].