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