The mouse brown locus encodes a putative membrane-bound metalloenzyme, tyrosinase-related protein-1 (TRP-1). We have examined the effect on mRNA expression of the locus of a number of mutant alleles. The common null mutant allele, brown, produces wild-type levels of TRP-1 mRNA, which is nonfunctional. Another recessive allele, cordovan-Harwell, has an intermediate, dark-brown phenotype and produces only very low levels of presumably normal TRP-1 mRNA. Two dominant alleles appear to act by killing the melanocyte in which they are expressed. One of them, Light, has normal size and amounts of TRP-1 mRNA. The other, White-based brown, produces no detectable TRP-1 mRNA. It has a gross DNA rearrangement at the 5' end, and we speculate that this results in activation of transcription of sequences not usually seen in melanocytes, and that this is toxic to the cell. The relationship between phenotype and molecular structure at the locus is discussed, and we draw some general principles applicable to other developmental genes.