In the last 3 years a group of inherited diseases, each with an underlying repeat expansion mutation, has been identified. Of the seven known trinucleotide repeat diseases three are neuromuscular disorders; myotonic dystrophy (DM), spinocerebellar ataxia type 1 (SCA1) and spinal and bulbar muscular atrophy (SBMA, also known as Kennedy’s disease). Clinically, these diseases are very different but they are linked by two features; slow progressive muscular wasting and anticipation. Anticipation is the phenomenon of increased symptom severity and decreased age of onset in succeeding generations and this correlates with an increase in the trinucleotide repeat number through generations. The other triplet repeat diseases are the fragile X syndromes of mental retardation (FRAXA and FRAXE), Huntington’s disease and dentatorubropallidoluysian atrophy (DRPLA).