To assess the cytotoxic activity of biparatopic DARPins in vitro, we treated a panel of ErbB2-overexpressing breast cancer cell lines (BT474, HCC1419, HCC2218, SKBR3, AU565 and ZR75-30) with wild-type PI3K activity and a range of EGFR, ErbB3 and PTEN expression levels (Fig. 2a). Here, ERBB3 is linked to breast carcinoma.