ESR1 and breast cancer: CCN5 expressing BC cells (e.g. MCF-7, BT-474, ZR-75-1, T-47D) are always ER-α positive (luminal type), while CCN5-negative cells are mostly triple-negative (ER-, PR- HER2-)-breast cancer (TNBC) cells (e.g. MDA-MB-231, MDA-MB-468, HCC-70, BT-20, MVT-1 and 4 T1) [38, 46, 52], which are enriched with tumor initiating cells (TICs)/cancer stem cells.