In this study, we showed, for the first time, that CAAs and CAFs express significantly higher levels of miR21 isomiRs than do ovarian cancer cells, and that the exosomal transfer of miR21 isomiRs from CAAs or CAFs to neighbouring cancer cells can increase the chemoresistance of these cells to paclitaxel through the downregulation of miR21's direct target APAF1 (Supplementary Fig. 8). The gene discussed is APAF1; the disease is ovarian cancer.