There is increasing indication that interspecific phenotypic differences result from variations in gene-regulatory interactions. Here we provide evidence that mice differ from zebrafish in the way they use homologous key components to regulate pigment cell differentiation. In both zebrafish and mice, one transcription factor, SOX10, controls the expression of another, MITF (microphthalmia-associated transcription factor), which in turn regulates a set of genes critical for pigment cell development and pigmentation. Mutations in either Sox10 or Mitf impair pigment cell development. In Sox10 -mutant zebrafish, experimentally induced expression of Mitf fully rescues pigmentation. Using lineage-directed gene transfer, we show that, in the mouse, Mitf can rescue Sox10 -mutant precursor cells only partially. In fact, retrovirally mediated, Sox10 -independent Mitf expression in mouse melanoblasts leads to cell survival and expression of a number of pigment biosynthetic genes but does not lead to expression of tyrosinase , the rate-limiting pigment gene which critically depends on both Sox10 and Mitf . Hence, compared with fish, mice have evolved a regulation of tyrosinase expression that includes feed-forward loops between Sox10 and tyrosinase regulatory regions. The results may help to explain how some embryos, such as zebrafish, can achieve rapid pigmentation after fertilization, whereas others, such as mice, become pigmented only several days after birth.