Аннотация:Introduction: Dysregulation of rRNA biogenesis is implicated in some human diseases. rDNA copy number (CN) in the genome is а factor at play. F-Sat III is an AT-rich repeat of pericentromeric heterochromatin in the first chromosome (lq l2). rDNA and lq 12 interact in interphase cells. rDNA CN might coпelate with f-Sat III.Materials and methods: W е analyzed leukocytes of 469 healthy donors (HD) and 777 patients with schizophrenia (F20.00, SP). CN was determined using non-radioactive hybridization with biotin-labelled DNA probes.Results: Genomes of SP contain increased levels of rDNA CN and decreased levels of f-Sat-III compared to HD: 532 ± 118 copies with 18,0 ± 5,9 pg/ng of f-Sat III and 413 ± 105 copies with 21,6 ± 6,9 pg/ng of f-Sat III, respectively, р < 10 -80. Amount of f-Sat III depends on rDNA CN:DNA of HD and SP containing low (200 - 350) or high (>700) CN of rDNA contained 10 - 30pg/ng of f-Sat III. 55% of SP have 500 - 600 CN of rDNA and 15-20 pg/ng of f-Sa tIII. Only 15% of HD have the same numbers. 16% of SP have <20 pg/ng of f-Sat III and rDNA CN > 500 with their ratio > 40, only 1.5% of HD have this ratio. 26% of HD have rDNA CN <400, f-Sat III> 20 pg/ng and their ratio <20 and only 1, 9% of SP are in this area. Thus, there is a coпelation between rDNA CN and the size if heterochromatin regions.Conclusions: SP have significantly less f-Sat III and more rDNA repeats compared to HD.