Why is it unwise to use only this dna sequence

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Why is it unwise to use only this dna sequence

You have the amino acid sequence of a protein and wish to search for the gene encoding for this protein in a DNA library. Using this protein sequence, you deduce a particular DNA sequence that can encode for this protein. Why is it unwise to use only this DNA sequence you have deduced as the probe for isolating the gene encoding your protein of interest from the DNA library?