A particular gene that you are studying is nearby a long sequence of noncoding repetitive DNA sequence that lies between your gene of interest and the centromere. This repetitive DNA sequence is flanked by several very similar sequences. Your gene of interest is expressed at moderate levels.
You find an odd mutant in which the repetitive DNA does not appear nearby your gene of interest.
As the repetitive DNA sequence was flanked by similar sequences, the most likely explanation for its absence is
With this long stretch of DNA removed, expression of your gene of interest in the mutant is likely