Marja Puurunen1, Andrew D Johnson2. 1. National Heart, Lung and Blood Institute's The Framingham Heart Study, Framingham, MA 01702, USA. 2. National Heart, Lung and Blood Institute's The Framingham Heart Study, Framingham, MA 01702, USA; National Heart, Lung and Blood Institute, Division of Intramural Research, Population Sciences Branch, Framingham, MA 01702, USA. Electronic address: johnsonad2@nhlbi.nih.gov.