Heterochromatin fraction called „constitutive” and euchromatin differ in respect of DNA types (tandem repeats vs coding/unique sequences) and in epigenetic modifications, i.e. in methylation of DNA, H3 and H4 histones and in acetylation of the latter two. These modifications result in permanent heterochromatin condensation. On the other hand, during the past decade, it has been well documented that gene expression in euchromatin is affected by its reversible condensation resulting from epigenetic modifications of DNA and H3 and H4 histones characteristic of heterochromatin with simultaneous histone modifications specific for euchromatin. These data imply that the term „heterochromatinisation” should bo replaced with „euchromatin condensation”.