異染色質 (heterochromatin) is a form of chromatin found in the nucleus of eukaryotic cells. It is tightly packed and generally transcriptionally inactive, meaning that the genes within this region are not expressed. Heterochromatin plays a crucial role in maintaining the structural integrity of chromosomes and regulating gene expression. There are two main types of heterochromatin: constitutive and facultative. Constitutive heterochromatin is always present and found in regions like the centromeres and telomeres, while facultative heterochromatin can change between active and inactive states depending on the cell's needs. Understanding heterochromatin is essential for studying genetics and cellular function.