1. Contribution from the Burnham Institute for Medical Research, 10901 North Torrey Pines Road, La Jolla, California 92037, NAST and Department of Environmental, Occupational, and Social Medicine and NAST and Department of Experimental Medicine and Biochemical Sciences, University of Rome Tor Vergata, Via Montpellier 1, 00133 Rome, Italy, and Institute of Occupational Medicine, Università Cattolica del Sacro Cuore, Largo Agostino Gemelli 8, 00168 Rome, Italy