Heterochromatin is a tightly packed form of chromatin found in the nucleus of eukaryotic cells. It is 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 in a compact form and is found in regions like the centromeres and telomeres, while facultative heterochromatin can change between a compact and a more relaxed state, depending on the cell's needs.