ESR1 and breast cancer: One group, designated “Cluster 1” (blue lines, Figure 1) included three ER-negative/HER2-positive BC cell lines, namely SKBR3, SUM190 and SUM225, which clustered with ER-positive/HER2-negative (T47D, HCC1428 and BT483) and ER-positive/HER2-positive (BT474, ZRF7530) cell lines.