Some studies have found that, in the process of ERα activation under the normal culture condition in ER-positive breast cancer cells, estrogen causes the endocytosis of ERα on the cell membrane to form endosomes, some of which enter the nucleus and cause transcriptional effect, and the others are degraded by lysosomes, leading to the gradual reduction of ERα level on the cell membrane. Here, ESR1 is linked to breast cancer.