Agouti protein has a short radius of action (Silvers and Russel 1955) and can be switched on and off during a single hair cycle (Bultman et al. 1992, 1994; Miller et al. 1993; Vrieling et al. 1994); thus, its regulated expression is thought to be responsible for the cream-colored or yellow ventral surface of mice carrying the black-and-tan (at) allele and for the yellow markings around the feet, ears, or head, i.e., tan points or head spots, of certain dog breeds.