ESR1 and breast cancer: We also examined the mRNA levels of HCN1‐4 in various breast cancer cell lines, including the estrogen‐receptor positive (MCF‐7, ZR‐75), HER2‐positive (BT‐474) and TNBC (MDA‐MB‐231, MDA‐MB‐453) cells compared with non‐neoplastic MCF‐10A breast epithelial cells.