Racing Thoroughbred horses are athletic and charismatic animals whose death or euthanasia in the public eye has a negative impact on the perception of the horse racing industry; owners, trainers, jockeys and veterinarians want to predict and prevent such catastrophes. Postmortem evaluation of racehorses has focused primarily on musculoskeletal injuries; however, horses also unexpectedly die suddenly on the track (sudden death, SD). Although cardiac conditions are frequently suspected as a cause of death, SD racehorses are often autopsy negative thus similar to autopsy-negative SD in humans which can be associated with inherited fatal arrhythmia syndromes. These disorders are associated with mutations in several large ion channel genes (channelopathies) identified by genetic analysis (molecular autopsy) which is largely unevaluated in the horse. We recently evaluated SD in New York (NY) and Maryland (MD) racehorses in a study comparing clinical information, postmortem evaluation including cardiac dissection, and cardiac conduction system histopathology. SD was 12% of racehorse submissions and most SD racehorses were autopsy-negative. While not excluding a genetic basis for SD, we also analyzed the genotypes of SD cases and controls which did not reveal significant differences in allele frequencies at any locus; however, this genome-wide association study (GWAS) was not restricted to autopsy-negative SD cases and was underpowered for a complex trait. Subsequent analysis restricted to Thoroughbred (TB) horses indicated several single nucleotide polymorphisms that approached statistical significance. Over the course of 2 years, we will investigate our hypothesis that genetic heart disease is associated with autopsy-negative sudden death in TB racehorses. AIM 1: DNA and cardiac tissue archiving: Because SD is a relatively rare trait, we propose first to build a multi-institutional and international SD repository of DNA isolated from autopsynegative SD cases and carefully-matched controls to support adequately powered genomic approaches. AIM 2: We will perform a molecular autopsy for SD by exome sequencing and in silico targeting of candidate cardiac channelopathy genes associated with human autopsy-negative SD as well as exploratory GWASidentified regions. This molecular investigation will be complemented by AIM 3 which will survey for genetic causes of SD in TB racehorses by expanding our exploratory GWAS to maximize our power to identify a locus at genome-wide significance. Understanding the genetics of SD in TB is crucial to preserving TB health, ensuring rider safety, and safeguarding the viability of racing industry. We have organized an international and multidisciplinary team of pathologists and geneticists to accomplish our goal of identifying genetic loci associated with SD in racehorses to guide identification of associated genes, inform test development and/or educate breeding decisions, and ultimately reduce SD prevalence.