[1] Johns Hopkins Univ, Sch Med, Dept Genet Med, Baltimore, MD 21205 USA
[2] Univ Florida, Coll Med, Dept Pediat, Gainesville, FL USA
[3] Univ Florida, Coll Med, Dept Obstet & Gynecol, Gainesville, FL 32610 USA
[4] Univ Florida, Dept Epidemiol, Coll Publ Hlth & Hlth Profess, Gainesville, FL USA
[5] Coll Med, Gainesville, FL USA