Mutations provide important structure–function relationships by allowing the correlation of phenotypes to the underlying genotypes. Knockout mutations that lead to loss-of-function are important and informative in this respect. However, spontaneous and induced mutations sometimes provide surprising phenotypes, which lead to unexpected functional insights and novel biochemical pathways, especially when multiple mutations (alleles) exist at a locus. An excellent example is provided by the microphthalmia (Mitf) locus in the mouse. The multiple Mitf alleles have their own phenotypic properties, most of which have been explained by the underlying mutation. However, one allele, the Mitf Mi-White (Mitf Mi-Wh) mutation, exhibits phenotypes that have not yet been fully explained. Here, the molecular, genetic, and phenotypic properties of this mutation are reviewed and an attempt made to explain the underlying biochemical reason for its observed effects.