MAPK3 and breast carcinoma: Lending support to a pro-mitogenic role of H2O2, H2O2 antagonized the cytostatic functions of TGFβ1 by the activation of Akt-ERK1/2-linked signaling.37 Although the precise target of H2O2 was not identified, many studies have shown that Akt-ERK1/2 are downstream mediators of Src and PTEN.34, 38 Similarly, the migration of MCF-7 breast cancer cells was enhanced by elevated Nox4 expression in a human mammary fibroblast cell line.36 Nox4 is known to predominantly produce H2O2 rather than O2−.