The DNA was collected from CDP participants from February 2006 to July 2007. For many participants, DNA collection took place at an annual follow-up visit. Following completion of phenotypic assessments, participants were invited to provide a DNA sample. However for some participants, especially for the considerable number who lived away from the main research offices, DNA was collected in the context of a special visit. The DNA was collected via saliva sample using Oragene collection kits under the supervision of a specially trained interviewer. Participants received an additional $20 for participating in the DNA collection portion of the project and provided separate consent for the genotyping component of the project. Saliva samples were subsequently labeled anonymously and mailed to Washington University in St Louis, Missouri, where DNA extraction and genotyping occurred. The DNA samples were obtained from 452 individuals, representing 93% of the target sample of regular CDP participants. The institutional review boards at all sites approved the study.
