A single nucleotide insertion mutation in a gene of phage T4 causes a shift in the reading frame of all subsequent downstream codons, and potential incorrect amino acids. The resulting protein is usually nonfunctional, and a T4 phage with such a frameshift mutation cannot reproduce on E. coli.

If three insertions occur in tandem, the reading frame is restored after the insertion. Such a mutation produces a protein with enough functionality to restore the phage's ability to infect E. coli. 12