In the experiment, proflavin-induced mutations of the T4bacteriophage gene, rIIB, were isolated. Proflavin causes mutations by inserting itself between DNA bases, typically resulting in insertion or deletion of a single base pair.
The mutants produced by Crick and Brenner could not produce functional rIIB protein because the insertion or deletion of a single nucleotide caused a frameshift mutation. Mutants with two or four nucleotides inserted or deleted were also nonfunctional. However, the mutant strains could be made functional again by using proflavin to insert or delete a total of three nucleotides. This proved that the genetic code uses a codon of three DNA bases that corresponds to an amino acid.
References
Crick FH, Barnett L, Brenner S, Watts-Tobin RJ. General nature of the genetic code for proteins. Nature. 1961 Dec 30;192:1227-32. PubMed: Entrez PubMed13882203.