ESR1 and breast cancer: In ER+ (MCF7s), HER2+ (SKBR3s) and triple negative (MDA-MB-231s) breast cancer lines, exposure to hypoxia or activation of HIF-1α signalling were shown to enhance secretion of exosomes carrying elevated levels of miRNA-210, which is believed to contribute to tumour progression by stimulating DNA repair, chemotaxis and survival of endothelial cells, thus enhancing tumour vascularisation (King et al., 2012; Fasanaro et al., 2008; Crosby et al., 2009).