ESR1 and breast cancer: Independently of estrogen receptor expression (ER+: MCF-7 and T-47D; ER-: MDA-MB-231) or epithelial/mesenchymal phenotype (epithelial-like low invasion: MCF-7 and T-47D; mesenchymal-like high invasion: MDA-MB-231), Nck1 protein levels in breast cancer cells were not consistent with the cancer stage.