Methylation activity of EZH2 facilitates heterochromatin formation thereby silences gene function. SETB1 is a histone methyltransferase that recruits HP1, thus inducing heterochromatin formation. Regional centromeres contain large amounts of DNA and are often packaged into heterochromatin. Fission yeast ("Schizosaccharomyces pombe") uses another mechanism for heterochromatin formation at its centromeres. Remodeling of chromosomal heterochromatin by EZH2 is also required during cell mitosis. This expansion causes epigenetic changes and formation of heterochromatin near the repeat. Reasons might be the presence of repetitive sequences, extensive DNA methylation, or heterochromatin. The regions that stain darker are regions of constitutive heterochromatin. When genes are placed near a region of constitutive heterochromatin, their transcription is usually silenced. Particularly, H3K9me3 is highly correlated with constitutive heterochromatin. Histone modifications are one of the main ways that the cell condenses constitutive heterochromatin. It is not repetitive and shares the compact structure of constitutive heterochromatin. The nucleus contains one to three prominent nucleoli and large amounts of dark-staining peripheral heterochromatin. Further compaction leads to transcriptionally inactive heterochromatin. This poor processing creates an abnormal nuclear morphology and disorganized heterochromatin.