Dynamic mutations due to trinucleotide repeat expansion are a new class of human genome mutations. CAG repeat expansion in the coding region of associated genes is the molecular genetic basis of the several diseases of nervous system. Eight DNA sequences with CAG repeats expressed in human brain were chosen from the GenBank database. The search of CAG expansion was carried out for patients with schizophrenia (brain and blood) and essential tremor. CAG repeat expansion has not been found for the loci. The distribution of allelic sizes is similar in the patients and control samples. Locus HS0073 has shown the polymorphism of the length for CAG repeat alleles. Statistically reliable excess of the homozygotes has been found for schizophrenic patients.