An early transposon (Etn) insertion has been identified as the mutation underlying the phenotype in muted mice. Analysis of genomic DNA revealed an insertion into the third intron. 183 bp of Etn sequence has been incorporated into the mRNA , likely by the activation of cryptic splice sites within the Etn. This results in an in-frame insertion into the translated protein at position 106, predicting a 246 amino acid protein. A significant reduction in mRNA levels is observed.