The study will use a case-control design nested within the VA Million Veteran Program (MVP), with genotype as the exposure variable and PTSD diagnosis (yes/no) as the outcome variable. The pool of potential PTSD cases will be identified initially based on self-report of a PTSD diagnosis on a previously completed self-report questionnaire collected in MVP, or evidence of a PTSD diagnosis in the VA electronic health record (EHR). Specific validation procedures will narrow the pool to confirmed PTSD cases and controls. Based on available MVP and VA EHR data, the investigators estimate a currently available source population of more than 11,000 confirmed PTSD cases among the approximately 145,000 MVP enrollees to date. By the time this project gets underway, the number of available cases (and controls) will be even higher, due to ongoing enrollment into MVP.