MAPK3 and cancer: Strikingly, it has been hypothesized that ERK1/2 is involved in the resolution of the transforming growth factor beta (TGF-β1) paradox, a phenomenon in which TGF-β1 functions as a tumor suppressor during the early stages of carcinogenesis by inhibiting cell proliferation and inducing apoptosis, but later shifts to promoting cancer cell invasion and spreading [79].