Additionally, DXR-responsive NP showed significantly greater binding to MDA-MB-231 cells than the non-responsive DXR-NP at the acidic extracellular pH of 6.5 (chosen to correspond to reported values of the pH of the tumor interstitium [10]), validating the use of the HER2-targeting responsive DXR-NP to selectively target low HER2-expressing cancer cells in the acidic tumor microenvironment, as we previously reported [14]. Here, ERBB2 is linked to cancer.