EN
The aetiology of variation in transcription of ribosomal genes is still an open question. Investigations carried out in many organisms showed that the variation depends not only on genetic mechanisms such as rDNA methylation, elimination of rDNA or the position effects. In this review, we discuss the results of the studies on repression of rRNA transcriptional activation from protein factors to conditions of cell culture, which may influence the variation of nucleolar organizer region activity.