ESR1 and breast cancer: However, metaplastic BC is more often diagnosed at advanced clinical stages due to larger primary tumors compared with other BCs and is associated with higher rates of chemoresistance, distant metastasis, and worse overall survival (OS).4,8 Metaplastic BC has a particularly high prevalence of triple-negative receptor status or the absence of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (HER2) amplification.9 Without targetable proteins, metaplastic BC remains challenging to treat.