Funding. This research was supported by the Netherlands Organization for Scientific Research (NWO) through the foundation of the Research Institute for Diseases of the Elderly, as well as grants from the National Institutes of Health (1PO1 AG17242–02), National Institute of Environmental Health Sciences (1UO1 ES011044), European Commission (QRTL-1999–02002), and the Dutch Cancer Society (EUR 99–2004). JRM was a fellow of the Damon Runyon Cancer Research Fund (DRG 1677).