ATR and acute lymphoblastic leukemia: Concerning RNR regulation, the observed consequences of ATR inhibition in T-ALL cells include the following: a decrease in RRM2 protein levels (Fig. 2c, d), reduced RRM1 protein levels (Fig. 2c, d), and a significant reduction in RRM2 pT33 (Fig. 2e), a CDK-mediated phosphorylation event which promotes RRM2 proteasomal degradation via interactions with the SCFCyclin F ubiquitin ligase complex44.