ESR1 and breast cancer: In the case of breast cancer cells, the human triple negative and highly osteotropic MDA-MB-231 cell line is likely the most employed, followed by the human T47D mammary ductal carcinoma cell line, while the epithelial like MCF7 cell line from mammary adenocarcinoma has a low ability for growth since these cells still express the estrogen receptor (ER); to foster in vivo growth animals are implanted with a 60-day slow release 17β-estradiol pellet (0.5 mg) before inoculation [47].