A new dominant white allele was suspected when two Thoroughbred horses with minimal white marking on the coat produced a colt with a large amount of coat depigmentation. Because of its association with similar patterns in other horses, the KIT gene was selected as a candidate gene, and all 21 exons were sequenced in the colt. A novel 5-bp deletion was discovered in exon 3 and was confirmed with allele-specific PCR. The mutation introduced a pre-mature stop codon, resulting in truncation of the protein. The deletion was not present in either parent and is suspected to be responsible for the extensive white coat colour in the colt. Additionally, a previously described missense mutation was detected in exon 14 of both the colt and sire but is not believe to be causative. Parentage testing was conducted as required by The Jockey Club for Thoroughbred registration, and the foal qualified for the stated parentage. This novel deletion in exon 3 is the 12th discovered dominant white allele in the horse.