The deletion of 3bp at codon positions 2642-2645 (delta 2642) of the gene 
mutated in Huntington's disease (HD) was analysed on the normal (N) and HD 
chromosomes of 79 French families affected with HD, and previously typed for the 
(CAG)n repeats. delta 2642 Polymorphism has been found over-represented on HD 
chromosomes, the relative risk of HD with the deletion being at a value of 8.26. 
In this study, the presence of the deleted allele on HD chromosomes increases 
the (CAG)n number (47.93 +/- 1.80 versus 43.50 +/- 2.78) and decreases the age 
of onset (41.34 +/- 2.09 versus 36.90 +/- 2.41) in the patients with versus 
without delta 2642; so the deletion may add to the severity of the disease. Our 
studies of delta 2642 on N chromosomes confirm that the deletion event occurs on 
N chromosomes with a (CAG)n allele length at the upper end of the normal size 
range.