CD44 and breast cancer: Previous studies have revealed that CD44 is highly expressed in triple‐negative breast cancer (TNBC) and correlated with poor survival of TNBC patients.42 Of note, CD44 also plays a key role in ER+ breast cancer, resulting in poor prognosis and radiotherapy resistance.42 However, the role of CD44 in drug resistance of ER+ breast cancer is worth studying in depth.