Inhibitors of checkpoint kinase 1 (CHK1),a central component of DNA damage and cell cycle checkpoint response, represent a promising new cancer therapy, but the global cellular functionsthey regulate through phosphorylationare poorly understood. To elucidate the CHK1-regulated phosphorylation network, we performed a global quantitative phosphoproteomics analysis, which revealed 142 phosphositeswhose phosphorylation levels were significantly different following treatment with the CHK1 inhibitor SCH 900776.