1. From the Division of Epidemiology and Institute of Human Genetics (L.D.A.), University of Minnesota, Minneapolis; the Department of Genetics (P.B.S., J.W.M.), Southwest Foundation for Biomedical Research, San Antonio, Tex; the Human Genetics Center (J.E.H.), University of Texas Health Science Center, Houston; and the Department of Medicine (M.P.S.), University of Texas Health Science Center, San Antonio.