HRG and cancer: Treatment of BT474 cells with epidermal growth factor (EGF) and heregulin (HRG), which are growth factors secreted by cancer cells [115–117] and activators of HER2 [118, 119], caused a significant increase in exosome production [40], leading to the possibility that enhanced secretion of exosomes may be a way for the tumor microenvironment to support cancer growth in the presence of therapeutic agents.