ERBB2 and gastric cancer: HER2‐positive gastric cancer cells resistant to trastuzumab emtansine discarded trastuzumab emtansine into the extracellular space by EV secretion (Le Joncour et al., 2019), and since a sufficient intracellular concentration of the cytotoxic payload DM1 is essential for the anti‐cancer activity (Barok et al., 2014), expulsion of DM1 may contribute to trastuzumab emtansine resistance (Figure 4).