ESR1 and breast cancer: Notably, we analyzed the expression of CTR1 in a panel of breast cancer cell lines and found that CTR1 was relatively high expression in triple‐negative breast cancer (TNBC) cell lines compared with estrogen receptor and progesterone receptor positive (ER+/PR+) or human epidermal growth factor receptor 2 positive (HER2+) cell lines (Figure 4a).