[Levels of chromatin condensation and activity]

The main progresses which occurred last years in the knowledge of chromatin, in its biochemical components, structure and organization levels, are briefly outlined. Inhibition of transcriptional activity is examined at the molecular, cellular and chromosomal levels. The different states of chromatin condensation according to physiopathological conditions are more particularly reviewed together with recent data on transcriptional regulation depending on histone and DNA modifications.