The National Cancer Institute (NCI) develops and supports a broad range of research relevant to screening for preneoplastic events and biological effects of carcinogen exposure. In cancer screening and detection, NCI supports studies on development of new methods for detecting cancer and/or preneoplastic states, systematic testing of new screening methods, and wider application of established methods. Recently, NCI developed clinical trials in chemoprevention and dietary intervention. These studies focus on populations identified by markers of exposure or markers of increased risk, and have as their end point reduction in cancer incidence.