During the past eight years a comprehensive genetic linkage map for the horse has been generated in Newmarket, through a fruitful collaboration between the Genetics section at the AHT and Professor Twink Allen at the TBA Equine Fertility Unit (Swinburne et al., 2000). In order to assess whether the map was adequate to start mapping disease traits in which we are interested, it was decided to attempt to map a clear, easily scoreable trait such as grey coat colour in the Thoroughbred horse. Grey coat colour is inherited as an autosomal dominant trait. Thoroughbred horses are fairly inbred, and as such they represent one of the more difficult horse breeds for genetic mapping. The majority of grey Thoroughbreds trace their ancestry back to Allcock's Arabian who was born in 1700, and horsemen have long noted an association between grey coat colour and the incidence of melanomas. It is hoped that characterizing the mechanism of grey coat colour formation might also lead to an improved understanding of the molecular basis of melanoma formation.