Genotype-phenotype correlations in a group of 100 patients with typical Friedreich ataxia (FRDA), and in three smaller clinically atypical groups (Arcadian FRDA, late-onset FRDA (LOFA), and FRDA with retained reflexes (FARR)), were studied at the Centre de Recherche Louis-Charles Simard, Service de Genetique Medicale, Service de Neurologie, Hopital Sainte-Justine, Departments of Genetics and Medicine, McGill University, Montreal General Hospital, and other centers. Almost all FRDA patients carry a GAA triplet repeat expansion on chromosome 9, despite phenotypic variation. Larger expansions lead to earlier onset and more severe disease. Arcadian FRDA and FARR were unrelated to expansion sizes. 
COMMENT. Testing for the GAA expansion is essential for the molecular diagnosis of the various forms of FRDA, and particularly atypical cases of late-onset, those with retained reflexes, and lack of the characteristic hypertrophic cardiomyopathy.