Mutation - Mutation position effect (Oct/13/2010 )

Hi all
I have a question;
We have generated a mutants with insertion in a gene with 4 exons. The insertion 1 in exon 2 and insertion 2 in exon 4. We got a mutant phenptype result with insertion 1(exon2) but with insertion 2 (in exon 4) no mutant phenotype (wild type result).
In general does the mutation or insertion position has an effect on the phenotype? please with example!
May I conclude that this gene is responsible for the phenotype or no?
please if you have documents about this matter send me!!

thank you very much for your cooperation!!
Marou

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