Inhibition of PI3Kδ-dependent Erk1/2 phosphorylation by PI3Kδ inhibitor serves as an efficient marker of its efficacy against childhood B-ALL since simultaneous targeting PI3Kδ and MEK1/2 may further improve the efficacy of PI3kδ inhibition. Here, MAP2K1 is linked to precursor B-cell acute lymphoblastic leukemia.