Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder 
caused by an expanded (CAG)n repeat on the huntingtin gene. It is characterised 
by motor, psychiatric and cognitive disturbances. Diagnosis can be confirmed by 
direct genetic testing, which is highly sensitive and specific and is now 
considered definitive. This study focused on 21 patients presenting with a 
clinical phenotype showing strong similarity to HD, but who do not have an 
expanded CAG in the huntingtin gene. However, other possible diagnoses could be 
evoked for most of them. Seven patients (3.5% of our cohort) could be considered 
as phenocopies of HD with no alternative diagnosis. Samples were screened for 
other triplet repeat diseases with similar presentation (DRPLA, SCA-1, SCA-2, 
SCA-3, SCA-6, and SCA-7) and were all negative. The repeat expansion detection 
technique (RED) was used to detect uncloned CAG repeat expansions and samples 
were also analysed by polymerase chain reaction for expansions of the 
polymorphic CAG-ERDA-1 and CTG18.1 trinucleotide repeats. RED expansion (>40 
repeats) was detected in only one patient. The results suggest that unstable 
CAG/CTG repeat expansions corresponding to known or unknown sequences are not 
involved in the aetiology of HD-like disorders. It is hypothesised that some of 
these phenocopies could correspond to mutations in other unidentified genes with 
other unstable repeats (different from CAG) or in unknown genes with other 
mutations.