Mitochondrial superoxide promotes cancer metastasis by increasing mitochondrial H2O2 levels and activating Src kinase, downstream intermediates of the TGF-β pathway and focal adhesion kinase family member Pyk2 (Porporato et al., 2014), the latter acting as a prometastatic effector (Wendt et al., 2013). Here, TGFB1 is linked to cancer.