ESR1 and breast cancer: Following an incubation with the purified proteins, both the single scFv and scFv fusion proteins were detected to bind strongly to HER‐2 high‐expressing breast cancer cells SK‐BR‐3 (ER‐/PR‐/HER‐2+) and BT‐474 (ER+/PR+/HER‐2+) cells (Figure 2).