ESR1 and metastatic malignant neoplasm: The two pairs of cell types are: (i) primary human mammary epithelial cells (HMECs) and their tumor-forming transformed variants, referred to here as R5 cells, which were generated by transduction of HMEC cells with expression vectors for SV40, hTERT and H-Ras [72], and (ii) the MCF10A non-tumorigenic immortalized human mammary epithelial diploid cell line [70], and the MCF7 breast cancer cell line, which is estrogen receptor positive (ER+), that was derived from a pleural effusion in a patient with metastatic cancer [73].