MAPK3 and systemic lupus erythematosus: In humans, it has been shown earlier that compared to cells from normal individuals and from patients with RA, MEK-catalyzed ERK-1/2 phosphorylation activity was significantly decreased in peripheral blood mononuclear cells and CD4+ T cells from patients with SLE despite the fact that normal levels of ERK were found (Deng et al., 2001).