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Mushroom is a yet to be mapped, theorized recessive dilution gene that fades red pigment in a horse's body hair coat. On a chestnut base coat the horse is born a pale beige with sometimes a greyish or pinkish tint and often keeps that color when it becomes an adult, but some turn darker when an adult.
On a bay base the horse is born a yellowish beige with a dark contrast stripe on the back, and as it grows older the horse will resemble a buckskin.
The dilution has appeared to date only in a few Shetland ponies in the United Kingdom. Originally suspected to be the silver dapple gene, several ponies with this colour have been tested for the extension gene and results have been negative, thus verifying that it appears to affect pheomelanin ("red" (chestnut)) pigment.