The human genome, made of about 3 billion bases, encodes between 75 and 100,000 genes. However, most of the genome is made of non coding sequences, whose function is still unknown. When a base variation occurs in a DNA fragment, base change, deletion or insertion of one or several bases, a mutation or a polymorphism is generated depending whether this base change modifies or not the function of the encoded gene. In 1991, a new type of mutation has been discovered, namely the expansion beyond a critical length of a three-base repeat, called triplet. These anomalies due to genome instability are not rare and are now responsible for at least twelve diseases. It is expected that other diseases due to the same mechanism will be discovered in the near future. This article illustrates the concept of mutation by triplet expansion and presents 3 diseases frequently observed in Pediatrics: the fragile X syndrome, myotonic dystrophy and Friedreich's ataxia.