A three-nucleotide insertion mutation. Template slippage on repeated sequences during replication can lead to small insertions and deletions. Shown here is a three-nucleotide (CAG) insertion in a repeated CAG region of the coding strand of a gene sequence, resulting in an additional Gln residue in a polyglutamine tract. The protein sequence shown is the sequence encoded by the coding strand of DNA, the strand that is transcribed into mRNA.