MAPK3 and breast cancer: Although p38γ activation may be essential in IR-induced G2/M arrest in HeLa and U2OS cells [19], studies from our laboratory and others have demonstrated that IR-induced ERK1/2 activation is necessary for the activation of the G2/M checkpoint response in MCF-7 breast cancer cells and that inhibition of ERK1/2 is associated with increased sensitivity to DNA-damaging agents [16,20,21].