 Each of these processes involves a DNA synthesis step in which strand slippage might occur leading to trinucleotide repeat expansion.The number of trinucleotide repeats appears to predict the progression, severity, and age of onset of Huntington's disease and similar trinucleotide repeat disorders.Other human diseases in which triplet repeat expansion occurs are fragile X syndrome, several spinocerebellar ataxias, myotonic dystrophy and Friedreich's ataxia.