Department of Preventive Medicine, USC Keck School of Medicine, Los Angeles, California 90089-9176, USA.|Department of Preventive Medicine, USC Keck School of Medicine, Los Angeles, California 90089-9176, USA.|Department of Preventive Medicine, USC Keck School of Medicine, Los Angeles, California 90089-9176, USA.|Department of Preventive Medicine, USC Keck School of Medicine, Los Angeles, California 90089-9176, USA.|Department of Preventive Medicine, USC Keck School of Medicine, Los Angeles, California 90089-9176, USA.