What nucleotide could be included in the sequence agatgcgagttacgg to form a genetic mutation?

The nucleotide sequence should be AGA TGC GAG TTA CGG (all upper case).

AGA TGC GAG TTA CGG (first strand) on DNA

TCT, ACG, CTC, AAT, GCC (complementary strand) to the above DNA

mRNA: UGU,UGC,GUG,UUA,CGG producing these amino acids:cytoseine, cytosine, valine and arginine. Adding an adenine could cause a mutation at the second codon (stop).