A cell-permeable, potent, and ATP-competitive inhibitor of Chk2 (checkpoint kinase 2; IC50 = 15 nM; Ki = 37 nM). Displays ~1,000-fold greater selectivity for Chk2 compared to Cdk1/cyclin B and CK1 (IC50 = 12 μM and 17 μM, respectively) and only weakly affects the activities of a panel of 31 kinases (< 25% inhibition at 10 μM), including Chk1. Shown to rescue both peripheral CD4+ and CD8+ T-cells from γ-irradiation-induced apoptosis with an EC50 of 3 μM and 7.6 μM, respectively. MW: 363.8. Purity ≥98% by HPLC