ERBB2 and breast cancer: Determining frequency and expression levels of HER2 on EVs released from confluent breast cancer cells into cell culture supernatants, we found a mean number of 1.4 × 103 HER2+EVs per μl and a median HER2-expression level of 3 × 104 fluorescence intensity in the supernatant of SKBR-3 breast cancer cells (Fig. 3h,i, suppl.