GENE-LEVEL MUTATIONS. “It may be a swapping of one nucleotide for another one, a deleting or omitting of a specific nucleotide from the strand, or an inserting of an extra nucleotide into the replication strand. Besides the single codon substitutions, on the level of codons there exist other categories of mutations.”
https://www.mdpi.com/2073-8994/12/8/1214/htm