Amutation in a gene in the fruit fly drosophila was found to affect eye color. the protein affected was found to be completely normal in its molecular weight and amino acid sequence, although much less of the protein is made in mutant flies. the most likely reason for the change in levels of protein is that the mutation

The mutation did not affect amino acid sequence or the molecular weight (no evidence of amino acid substitution or protein truncation). Therefore, the mutation is not likely within the coding region of the gene.  Most likely the mutation is in the regulatory region of the gene such as the promoter region. Such mutation would interfere with transcription initiation and hence decreases the amount of mRNA and protein produced.