The mFISH studies allowed multicolor detection of numerous targets in interphase nuclei of non dividing neu­ronal cells, obtained from the fetal brain (Fig. 1). The technique applied in the present study provides accurate identification of individual chromosomes with application of the original collection of DNA probes, and to success in investigating the chromosomal variations in the fetal hu­man brain. Interphase FISH signal scoring was performed using 1,000 nuclei per sample of the fetal brain. The fre­quencies of aneuploidies involving different chromosomes (numbers of chromosomal aneuploidies per nuclei in per­cent) were determined. The results are present in Table 1. There is high variability in the frequencies of aneuploidies between samples of fetal brains and different chromo­somes in individual brains. The frequencies of chromo­somally abnormal nuclei (nullesomy, monosomy, trysomy and tetrasomy) varied from 0.2 to 15.6% for different chromosomes. We have not found any specific aneuploi­dies characteristic for individual fetal brain samples. The spectrum of aneuploidies was non specific: different nuclei usually have one or very rarely two (or more) chromo­somal simultaneous abnormalities. The mean frequencies of aneuploidies determined for 10 samples of fetal brain are 5% for chromosome 1; 3.7% for chromosomes 13 and 21; 1.8% for chromosome 18; 5.8% for chromosome X and 5.7% for chromosome Y. More common aneuploidies were associated with aberrations involving sex chromo­somes and chromosome 1. The sum of the frequencies of aneuplodies (or mean cumulative frequency) involving chromosomes 1, 13, 18, 21, X and Y was 22%.