Non sense mutation - (Nov/11/2010 )

Hi
We tried to confirm the function of a gene having a nonsense mutation in the first exon. We did an insertional mutation in exon 2 and exon 4. The result that we find a wild type phenotype!
NB: The gene has 2 domains, one in the 1st exon, the other in the 2nd, 3rd and 4th exon.
Does the position of mutation has an effect on the phenotype?
Or does the nonsense mutation has no effect? if yes how could that happen?

Thank you for your help

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