What is "trinucleotide repeat disorder"

n. (context genetics medicine English) Any of a group of disorders caused by stretches of DNA that contain a sequence of three nucleotides repeated many times

Trinucleotide repeat disorders (also known as trinucleotide repeat expansion disorders, triplet repeat expansion disorders or codon reiteration disorders) are a set of genetic disorders caused by trinucleotide repeat expansion, a kind of mutation where trinucleotide repeats in certain genes exceed the normal, stable threshold, which differs per gene. The mutation is a subset of unstable microsatellite repeats that occur throughout all genomic sequences. If the repeat is present in a healthy gene, a dynamic mutation may increase the repeat count and result in a defective gene.

Trinucleotide repeats are sometimes classified as insertion mutations and sometimes as a separate class of mutations.