RÉSUMÉ
<p><b>OBJECTIVE</b>To investigate the normal range of (CAG)n in spinocerebellar ataxia type 1 (SCA1) gene and spinocerebellar ataxia type 3 (SCA3/MJD) gene in 110 normal subjects of Han population in Northeastern China, to assess the genotypes for clinically diagnosed spinocerebellar ataxia(SCA) individuals including 25 patients from 8 families and 6 sporadic patients, and to make presymptomatic and prenatal diagnosis.</p><p><b>METHODS</b>DNA fragments from the normal subjects and the patients were detected by fluorescence-PCR. Homozygosities were selected for DNA sequencing.</p><p><b>RESULTS</b>The normal ranges of (CAG)n of SCA1 and SCA3/MJD were 20-39 and 14-38 repeats respectively, SCA1 was found mostly to be 26 and 27 repeats, allele frequency 34.09% and 20.91%; heterozygosity was 84.55%, SCA3/MJD was found mostly to be 14 repeats, allele frequency 39.55%, heterozygosity was 78.18%.(CAG)(68) of SCA3/MJD gene of one affected individual had been found in a family but no CAG mutative expansion in related members was observed.</p><p><b>CONCLUSION</b>The normal ranges of CAG repeats vary with areas and races. SCAs genotyping is the first choice in presymptomatic and prenatal diagnosis.</p>