Insertions and deletions can lead to frameshift mutations. (a) A reading frame in the DNA begins with the start codon, ATG, and stops with a stop codon (TAG in this case). When the coding strand of the DNA is transcribed into mRNA, these sequences are AUG (start) and UAG (stop). (b) Insertions or deletions of one or two base pairs in a protein-coding sequence result in frameshift mutations, in which the reading frame becomes out of register relative to the wild-type sequence and typically leads to a premature stop codon.