ESR1 and breast cancer: Studies on breast cancer cell lines demonstrated an overexpression of SDC4 in MDA-MB-231 triple-negative breast cancer (ER-negative, PR-negative, and HER2-negative) and HER2-overexpressing SKBR3 (ER-negative, PR-negative, HER2-positive) cells, but a downexpression in less aggressive MCF-7 (ER-positive, PR-positive, HER2-negative) cancer cells [46,47].