Seymour Benzer in the late 1950s had developed an assay using phage mutations which provided the first detailed linearly structured map of a genetic region. Crick felt "he" could use mutagenesis and genetic recombination phage to further delineate the nature of the genetic code. In the Crick, Brenner et al. experiment, using these phages, the triplet nature of the genetic code was confirmed. They used frameshift mutations and a process called reversions, to add and delete various numbers of nucleotides. When a nucleotide triplet was added or deleted to the DNA sequence the encoded protein was heavily affected. Thus, they concluded that the genetic code is not a triplet code because it did cause a frameshift in the reading frame. They correctly concluded that the code is degenerate (multiple triplets can correspond to a single amino acid) and that each nucleotide sequence is read from a specific starting point.