Several mutations and genes have been identified that affect the pattern of hair follicle development, but Tbx15 is the only gene of which we are aware that affects the pattern of hair pigmentation in different body regions. Ventral areas that normally produce yellow hair in the trunk, limbs, and craniofacial regions are expanded in deH/deH mice and, in the trunk at least, represent inappropriate dorsal expression of an Agouti mRNA isoform that is normally restricted to ventral skin. The deH allele is caused by a large deletion that removes most of the Tbx15 coding sequence, but the pleiotropic phenotype is caused by a simple loss of function for Tbx15 rather than a dominant-negative or contiguous gene effect. In particular, there is no heterozygous phenotype, no other genes lie within or close to the deletion breakpoints, and the expression pattern of Tbx15 is consistent with the spectrum of phenotypic abnormalities in both the original de allele and the deH allele. Finally, a Tbx15 targeted allele has the same phenotype as deH.