A recessive, fully penetrant mutation (c(m1OR)) at the mouse albino locus that results in coat-color mottling has been characterized at the molecular level. Restriction mapping and DNA sequencing analyses provide evidence that mutants carry a 5.4-kb intracisternal A particle (IAP) element insertion upstream of the tyrosinase (Tyr) promoter. Northern blot analysis and reverse transcription-PCR results show that the tyrosinase gene is expressed at much lower levels in mutant than in wild-type mice. The mutant Tyr gene still retains the tissue-specific expression pattern, and the Tyr transcript is not initiated from the IAP long terminal repeat promoter. We propose that the IAP insertion isolates the promoter of the tyrosinase gene from upstream cis-acting regulatory elements, leading to a substantially decreased level of Tyr gene expression in mutants.