ESR1 and breast cancer: Among them, luminal B remains to be the most common subtype in young women, accounting for 15%–20% of the total breast cancer cases, and within luminal B, ESR1-CCDC170 fusion-positive subtype, constituting 6% to 8% of the luminal B class, persists to be the most dominant subtype [3,4,5,6,7,8,9].