• About This Guide
  
• How This Guide is Being Developed
  
• Philosophy of this guide
  
• Using this Guide
  
• Thinking Scientifically
  
• About the Author
  
• Documented Genes
  
• What is a documented gene?
  
• Black/Red (MC1R)
  
• Bay (ASIP)
  
• Cream/Pearl (SLC45A2)
  
• Dun (TBX3)
  
• Gray (STX17)
  
• Silver (PMEL17)
  
• Champagne (SLC36A1)
  
• Roan/Tobiano/Sabino/White Spotting (KIT)
  
• Splash White (MITF/PAX3)
  
• Leopard Complex (TRPM1/ECA3P)
  
• Frame (EDNRB)
  
• Tiger Eye (SLC24A5)
  
• Height Regulation (HMGA2/LCORL)
  
• Mushroom (MFSD12)
  
• Hypothetical Genes
  
• What is a hypothetical gene?
  
• Borrowed Genes
  
• Glossaries
  
• Glossary of Colors
  
• Glossary of Terms
  

Gray (STX17)

• Last updated: 2026-01-14

  Common Names: Gray

  Scientific Name: Syntaxin-17 (STX17)

  Equine Chromosome: 25

  General Overview:

  The gray mutation causes hyperpigmentation (excessive amounts of pigment). Foals are born darker than normal, and will progressively develop white hair with age. Gray is caused by a duplication in the STX17 gene, resulting in two copies of the gene being found on one chromosome (G2 for gray x 2). Many gray lines have an additional duplication of this gene resulting in three copies of the gene on a single chromosome (G3 for gray x 3). [1]

  
  

  The more duplications a horse has the faster they will turn white and the higher their risk of cancer.[2]

  
  

  Fleabites are caused by areas of pigment reactivating as a horse ages. They are almost always found on heterozygous gray horses and are more common in G2 lines.[2]

  
  

  Bloodmarks are large areas where the gray gene was delayed in activating. They tend to be visible from a young age and will often gray eventually just at a much reduced rate. My personal conjecture is these are probably caused by areas where one of the STX17 duplications was disabled early on during embryonic development. Effectively a somatic mutation.

  
  

  Chubari spots are large coin sized circular areas that gray faster than other areas.

  
  

  HorseGeneticsGame.com currently only uses one gene to represent gray.

• Citations:

  1. Rubin, CJ., Hodge, M., Naboulsi, R. et al. An intronic copy number variation in Syntaxin 17 determines speed of greying and melanoma incidence in Grey horses. Nature Communications 15, 7510 (2024). doi: 10.1038/s41467-024-51898-2
  2. Andersson L. White horses - non-coding sequences drive premature hair greying and predisposition to melanoma. Ups J Med Sci. 2024 Apr 2;129. doi: 10.48101/ujms.v129.10626. PMID: 38571883; PMCID: PMC10989212. https://pmc.ncbi.nlm.nih.gov/articles/PMC10989212/

• Examples:

  A G2 gray at ages 0, 6 and 12

  

  
  

  A G3 gray at ages 0, 6 and 12