Relative phosphorylation levels of MAP kinases and MAP kinase kinases in human cell lines. Western blotting was performed to quantify the relative phosphorylation of ERK1 (yellow), ERK2 (blue) and MEK1 or MEK2 (purple) at their activation sites in subconfluent cultures of proliferating cells. MEK1 and MEK2 cannot be distinguished with the antibody used. Values are the means of at least triplicate (range 3 to 54) determinations for measurements of the phosphorylated forms of the kinases in 69 diverse human cell lines analyzed with 588 lysates by Kinetworks™ Phospho-Site Screen (KPSS) multi-immunoblotting . Cell lines have been divided into groups on the basis of their organ of origin.