PERP and breast cancer: To test whether, in addition to PERP, levels of other proteins in EVs emitted by breast cancer cells potentially correlate with their trastuzumab response, we used liquid chromatography-tandem mass spectrometry (LC-MS/MS) [22] to perform an initial screen of the proteomes of EVs emitted by BT-474 and BT-474TR cells treated or not with trastuzumab.