Trinucleotide repeats are very common and widely spread throughout the human genome. They can be found inside introns or even inside open reading frames (exones, in translated and untranslated regions). They can be very unstable and cause expansions or deletions during DNA replication. Expansions occur during replication of the lagging strand, when a hairpin- formed by the trinucleotide repeat-is present at an Okazaki fragment. The presence of the hairpin on a template strand mainly leads to deletions. Trinucleotide repeat expansions are a couse of many diseases, known as trinucleotide expansion diseases, such as the fragile X syndrome (CGG), Huntington's disease (CAG) or Myotonic distrophy (CTG).