MAPK3 and acute lymphoblastic leukemia: Three of four examined ALL cell lines (SEMK2, 697, CCRF-CEM) exhibited baseline phosphorylation of ERK1/2 and p90RSK, which rapidly and markedly decreased after incubation with MEK1/2 inhibitor, selumetinib (SEL), approved for treatment of melanoma and tested in other solid malignancies (Fig 1B).