The Huntington's chorea mutation consists of expansion of trinucleotide CAG repeats in the recently discovered gene IT-15. In this work, for the first time in a population of Russian patients, correlations between the number of copies of CAG repeats and various clinical characteristics of the disease are investigated. It is established that the degree of triplet expansion determines the age of onset of the disease and the rate of progression of the neurological and mental symptoms of Huntington's chorea, and it is also shown that the genetic instability of the mutant allele is considerably higher upon transmission of the disease gene along the paternal line. We obtained direct confirmation of the possibility of genetic instability of a normal allele inherited paternally. In this work, the first successful direct (including preclinical) DNA diagnosis in Russia of Huntington's chorea was obtained.