We evaluated the association between age at onset of Friedreich's atax
ia and alleles of two restriction fragment length polymorphisms (RFLP)
at D9S15 and D9S5 in the 9q13-9q21.1 region. We studied 65 Italian pa
tients from 49 families. Age at onset was not normally distributed in
our patients, suggesting allelic heterogeneity. Patients homozygous fo
r allele 1 of MspI RFLP detected by probe MCT112 at D9S15 (M1) had an
earlier onset (mean 9.3, SD 3.4 years) than patients homozygous for al
lele 2 (M2; mean 12.1, SD 4.3). Heterozygotes had an onset age similar
to that of the M2 homozygotes. These findings suggest that the M1 all
ele might be a marker of one allelic early-onset Friedreich's ataxia m
utation.