noun, plural: trinucleotide repeats
A form of mutation characterized by a stretch of three nucleotides (codon) repeated in multiple times in the DNA sequence.
The trinucleotide repeats expand in successive generations and their effect depends on their length in the DNA sequence. When the extent of trinucleotide repeats exceeds the stable threshold, it manifests into a disease. A set of disorders related to trinucleotide repeat expansion is called trinucleotide repeat disorders.