Test for progressive greying (grey). It detects the intronic duplication in STX17 responsible for the horse being born pigmented and progressively losing pigment until it becomes white with age. It also reports the allele (G2 or G3) associated with the rate of greying and the risk of melanoma. Complementary to, not a substitute for, clinical examination.

Applicable breed: many breeds (Lipizzan, Camargue, Purebred Spanish Horse, Quarter Horse, etc.). Frequencies: limited data; the proportion of G2 versus G3 varies by breed.

• Progressive greying with age: the foal is born with normal colour and progressively loses pigment until it becomes white, retaining dark skin.
• Depigmentation marks (vitiligo) and mottling of the skin.
• It is a coat phenotype, not a disease; in older horses it is frequently associated with dermal melanomas.

The Grey locus was mapped to chromosome 25. In 2008 Rosengren Pielberg and colleagues described a 4.6 kb duplication in STX17 associated with greying and melanoma; in 2024 Rubin and colleagues demonstrated two alleles according to copy number, G2 and G3.

• Do not pair G3/G3 if you wish to reduce the risk of melanoma; consider the G2/G3 genotype in selection.
• Intensive selection for white (G3) increases the frequency of the higher-risk allele.
• Do not confuse greying (STX17) with other congenital white patterns.
• A negative result does not exempt from clinical surveillance in horses that are white for other reasons.

Monitor the appearance of dermal melanomas (parotid, tail, genitalia) in aged grey horses. Differential diagnosis with albinism and with other white patterns. The duplication is a single-locus trait with a pleiotropic effect on pigmentation and melanocytic neoplasia.

1. Rosengren Pielberg 2008, A cis-acting regulatory mutation causes premature hair graying and melanoma (PMID 18641652); OMIA:001356-9796.
2. Rubin 2024, Intronic copy number variation in STX17 determines speed of greying and melanoma (PMID 39209879); OMIA:001356-9796.