@srpearce 

From Edit a Gene Part B:

Make a single base pair substitution mutation in the gene below that results in a protein that is longer than the protein produced by the original gene. When you are satisfied with your change and its effect, click the “SUBMIT” button.

Note that a "single base pair substitution mutation" is when a single base is changed to another base; for example, changing the A at position 80 to a T. Deletions and insertions are not allowed.