ESR1 and breast cancer: Purified rfhSP-D (5 μg/ml) bound breast cancer cell lines, triple-negative BT20 (ER−/PR−/HER2−), triple-positive BT474 (ER+/PR+/HER2+), and HER2-positive SKBR3 (ER−/PR−/HER2+) (Figure 1D).