ERBB2 and breast cancer: To test whether the high baseline fraction of intrinsically immobile HER2 was restricted to cell lines with extremely strong HER2 expression, we compared the results from BT-474 breast cancer cells (which strongly overexpress HER2) to those obtained from our TREx-HEK293 cell line without induction of HER2 overexpression, which results in intermediate expression levels (Supplementary Table 1 and Supplementary Fig. 2).