Agents now come to the U.S. National Cancer Institute (NCI) from many sources for preclinical evaluation and/or potential development (1). In most cases, experimental agents have limited antiproliferative data against a broad spectrum of human cancers, and these agents usually are then tested in the NCI’ s in vitro anticancer drug screen. Data from the screen permits the identification of agents that exhibit differential activity among multiple tumor cell line panels and/or that exhibit patterns of in vitro anticancer drug activity that may correspond to novel molecular targets (2). Agents that exhibit differential or novel patterns of in vitro activity are subsequently tested in vivo using the hollow fiber assay to assess their potential for in vivo activity in minimum challenge models. Some agents that cause differentiation, inhibit angiogenesis, and/or work in combination with other experimental or known anticancer agents are also submitted to NCI for preclinical evaluation. Such agents often require alternative and/or specialized in vitro or in vivo evaluation procedures to confirm initial experimental findings and to further support the rationale for development of potential new drug therapies.