A p53 peptide was used to measure DNA-PKcs kinase activity in the presence of M3814. DNA-PKcs (20 nM) was incubated with p53 peptide (250 μM), 1 μM dsDNA, and 80 nM [γ−32P] ATP at37 °C for 10 min. in 25 mM Tris−HCl (pH 7.5), 10 mM MgCl2,10 mM DTT, and 5 % sucrose. Inhibition was measured by comparing the amount of P-p53 in each sample relative to the DMSO control (Lane 3).