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An autosomal recessive missense mutation in glutamate metabotropic receptor 6 (<jats:italic>GRM6<\/jats:italic> c.533C&gt;T, p.(Thr178Met)), called <jats:italic>CSNB2<\/jats:italic>, was previously identified in one Tennessee Walking Horse and predicted to reduce binding affinity of the neurotransmitter glutamate, impacting the retinal rod ON\u2010bipolar cell signaling pathway. Thus, the first aim was to identify the allele frequency (AF) of <jats:italic>CSNB2<\/jats:italic> in breeds with reported cases of CSNB and breeds closely related to the Tennessee Walking Horse. The second aim was to perform ocular examinations in multiple breeds to confirm the link between genotype and CSNB phenotype. In evaluating 3518 horses from 14 breeds, the <jats:italic>CSNB2<\/jats:italic> allele was identified in nine previously unreported breeds. The estimated AF was highest in pacing Standardbreds (0.17) and lowest in American Quarter Horses (0.0010). Complete ophthalmic examinations and electroretinograms (ERG) were performed on 19 horses from three breeds, including one <jats:italic>CSNB2<\/jats:italic> homozygote from each breed. All three <jats:italic>CSNB2\/CSNB2<\/jats:italic> horses had an electronegative ERG waveform under scotopic light conditions consistent with CSNB. The remaining 16 horses (seven <jats:italic>CSNB2\/N<\/jats:italic> and nine <jats:italic>N\/N<\/jats:italic>) had normal scotopic ERG results. All horses had normal photopic ERGs. This study provides additional evidence that <jats:italic>GRM6<\/jats:italic> c.533C&gt;T homozygosity is likely causal to CSNB in Tennessee Walking Horses, Standardbreds, and Missouri Fox Trotting Horses. Genetic testing is recommended for breeds with the <jats:italic>CSNB2<\/jats:italic> allele to limit the production of affected horses. This study represents the largest across\u2010breed identification of CSNB in the horse and suggests that this disorder is likely underdiagnosed.<\/jats:p>","DOI":"10.1111\/vop.13151","type":"journal-article","created":{"date-parts":[[2023,10,12]],"date-time":"2023-10-12T03:37:34Z","timestamp":1697081854000},"page":"248-255","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":5,"title":["Additional evidence supports <scp><i>GRM6<\/i><\/scp> p.<scp>Thr178Met<\/scp> as a cause of congenital stationary night blindness in three horse breeds"],"prefix":"10.1111","volume":"27","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-6238-9793","authenticated-orcid":false,"given":"Elizabeth","family":"Esdaile","sequence":"first","affiliation":[{"name":"Veterinary Genetics Laboratory School of Veterinary Medicine University of California, Davis  Davis California USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8671-6661","authenticated-orcid":false,"given":"Kelly E.","family":"Knickelbein","sequence":"additional","affiliation":[{"name":"Department of Clinical Sciences College of Veterinary Medicine Cornell University  Ithaca New York USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1503-1419","authenticated-orcid":false,"given":"Callum G.","family":"Donnelly","sequence":"additional","affiliation":[{"name":"Department of Clinical Sciences College of Veterinary Medicine Cornell University  Ithaca New York USA"},{"name":"Department of Population Health and Reproduction School of Veterinary Medicine University of California, Davis  Davis California USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Michelle","family":"Ferneding","sequence":"additional","affiliation":[{"name":"Department of Surgical and Radiological Sciences School of Veterinary Medicine University of California, Davis  Davis California USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Monica J.","family":"Motta","sequence":"additional","affiliation":[{"name":"Department of Surgical and Radiological Sciences School of Veterinary Medicine University of California, Davis  Davis California USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9771-9294","authenticated-orcid":false,"given":"Brett D.","family":"Story","sequence":"additional","affiliation":[{"name":"Department of Surgical and Radiological Sciences School of Veterinary Medicine University of California, Davis  Davis California USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7566-5969","authenticated-orcid":false,"given":"Felipe","family":"Avila","sequence":"additional","affiliation":[{"name":"Veterinary Genetics Laboratory School of Veterinary Medicine University of California, Davis  Davis California USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5924-0234","authenticated-orcid":false,"given":"Carrie J.","family":"Finno","sequence":"additional","affiliation":[{"name":"Department of Population Health and Reproduction School of Veterinary Medicine University of California, Davis  Davis California USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7771-9166","authenticated-orcid":false,"given":"Brian C.","family":"Gilger","sequence":"additional","affiliation":[{"name":"Department of Clinical Sciences North Carolina State University  Raleigh North Carolina USA"},{"name":"Department of Ophthalmology School of Medicine University of North Carolina  Chapel Hill North Carolina USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5852-9308","authenticated-orcid":false,"given":"Lynne","family":"Sandmeyer","sequence":"additional","affiliation":[{"name":"Department of Small Animal Clinical Sciences Western College of Veterinary Medicine University of Saskatchewan  Saskatoon Saskatchewan Canada"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5617-9677","authenticated-orcid":false,"given":"Sara","family":"Thomasy","sequence":"additional","affiliation":[{"name":"Department of Surgical and Radiological Sciences School of Veterinary Medicine University of California, Davis  Davis California USA"},{"name":"Department of Ophthalmology &amp; 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