MAPK3 and cancer: Another example of an ATOX1-regulated signaling pathway involved in the control of excessive proliferation of cancer cells is the RAS-RAF-MEK-ERK cascade as mitogen-activated protein kinase kinase (MEK) has been identified as a copper-dependent kinase, whereas copper is an allosteric regulator of the interaction between MEK and extracellular signal-regulated kinase 1/2 (ERK1/2) [81, 82].