FER and breast cancer: Here we identify the tyrosine kinase FER as the kinase that phosphorylates PKCδ on Y374 and show that phospho-Y374-PKCδ stabilizes a normally transient RAB5-RAB7–positive endosome population to shift the balance from RTK degradation to recycling, with potential consequences for the survival and growth of a subset of triple-negative and HER2+ breast cancers.