ESR1 and breast carcinoma: Immunoblot analysis of human breast cancer cell lines MCF7 and T47D, which are known to be ERα positive, with the HC-20 antibody raised against the C-terminus of human ERα showed that these cell lines expressed both the 67-kD wt ERα (arrowhead in Fig. 7A) and a protein estimated as the 61-kD (arrow in Fig. 7A).