During a genomewide association study, Brosnahan et al. found a single-nucleotide polymorphism (SNP) in an intron of the tetraspanin 9 gene, located on chromosome 6, when they compared horses with a fever and EHM with matching horses that had only developed a fever during experimental infection/natural EHV-1 outbreak conditions. The gene discussed is TSPAN9; the disease is Fever.