ESR1 and breast cancer: Compared to HME cells, geminin, TopoIIα, Cdc7 and CKIε proteins are overexpressed in several estrogen receptor (ER)-positive breast cancer cell lines (for example, MCF7 and BT474) as well as ER-negative breast cancer cell lines (for example, MDAMB231, MDAMB453 and SKBR3) (Figure 5A).