The development of anticancer drugs has relied primarily on two traditional approaches. Synthetic or natural compounds are routinely screened for anticancer activities using a cell-based assay. The National Cancer Institute (NCI) has relied on a panel of human tumor cell lines to search for compounds that inhibit cell growth (1 ,2 ). Inhibitors identified in this screen are then further characterized for toxicity and antitumor activity in animal models. A strategy that is highly popular in industry is high-throughput screens for compounds that inhibit the in-vitro activities of specific enzymes or proteins (kinases, phosphatases, etc.) (3 ). This approach relies on the establishment of a robust in-vitro assay for the protein of interest. Although kinases are the preferred substrates because many of the existing chemical libraries were designed to identify kinase inhibitors, screens for other cellular targets are limited only by the development of an appropriate assay.
