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The KIT gene in horses is responsible for many different white patterns. It has four known white pattern alleles: roan, tobiano, sabino, and dominant white. There are many more white traits which are believed to be on KIT, but which haven't yet been located. Despite being very different traits, because these are all alleles of the same gene, only two of them can be present at a time in one horse. This also means that if a horse is homozygous for any of these traits, it will have none of the others.
The wild-type recessive allele of KIT results in no white patterning. Because there are so many different allelic traits on KIT, the recessive allele has many different notations used for it. When referencing all KIT traits together it can be written as "n" for "none" or a plus sign "+". When referencing just one KIT trait, it is often written as a lower case version of that trait followed by a plus sign, such as "rn+" or "to+".
The KIT gene is closely positioned on its chromosome near to extension, which is responsible for black color. This means that the two genes are "linked" and they will have their alleles only passed on in pairs. For example, if a horse is heterozygous for both extension and tobiano, it will always pass on the same extension-tobiano allele pair. It could always result in solid colored chestnuts and black tobianos, or it could always result in tobiano chestnuts and solid black horses. Determining which KIT alleles are linked to which extension alleles is an important step in breeding KIT horses.
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