ESR1 and neoplasm: The three cell lines examined (MCF-7, SKBR3 and MDA-MB-231) represent different tumour phenotypes: MCF-7 are estrogen receptor (ER), progesterone receptor (PR), epidermal growth factor (HER2) positive and considered mildly invasive; SKBR3 are ER/PR negative but HER2 over-expressing and possess an invasive phenotype ; MDA-MB-231 represent a triple negative, highly invasive phenotype31.