A replacement of a letter with different letter would indicate a missense mutation. (eg. GGA would become something like GCA and it would make something completely different).
Explanation:
A missense mutation is when the RNA replicates the instructions wrong from the DNA. When RNA is transcribed to make amino acids, it will make a completely different amino acid rather than the one that is needed/blueprinted. For example, if the sequence was AAG in the RNA (Derived from TTC DNA), it would create Lys amino acid. However, if the RNA sequence was AGG (Derived from abnormal TCC DNA), it would make Arg amino acid, which is different.
