ESR1 and triple-negative breast carcinoma: Experiments using triple-negative patient-derived xenografts (PDX), MDA-MB-231 triple negative breast cancer cells orthotopically inoculated in immunocompromised mice and FVB/N mice bearing orthotopically transplanted tumors from MMTV-PyMT; Pdgfc+/+ or MMTV-PyMT; Pdgfc−/− mice were shown to upregulate ERα expression when PDGF-CC was downregulated or inhibited and consequently respond to tamoxifen.