LY2603618 (IC-83) is a highly selective Chk 1 inhibitor with potential antitumor activity. LY2603618 (IC-83) binds to and inhibits the activity of chk1, which may prevent the repair of DNA caused by DNA-damaging agents, thus potentiating the antitumor efficacies of various chemotherapeutic agents. Chk1, an ATP-dependent serine-threonine kinase, is a key component in the DNA replication-monitoring checkpoint system and is activated by double-stranded breaks (DSBs); activated chk1 is overexpressed by a variety of cancer cell types. [1][2]