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.