EIF4EBP1 and acute lymphoblastic leukemia: In order to determine whether the heterogeneity of these antiproliferative and proapoptotic responses were associated with differential effects on PI3K signaling, we examined the phosphorylation levels of AKT, S6 and 4E-BP1, all of which are downstream of PI3K, in 5 ALL-LTCs representing different genetic subtypes of ALL.