ATM and cancer: As another control to test the specificity of Nutlin-3 in the assay which measures induction of PD-L1 in p53-wt cancer cells, we also evaluated the ataxia-telangiectasia mutated (ATM) kinase inhibitor KU55993, which inactivates p53 [51] and therefore has the opposite effect of Nutlin-3 on p53 pathway activity.