a chromosomal region that stains darkly when treated with dna-binding chemicals; it is more condensed.a chromosomal region that does not stain as darkly when treated with dna-binding chemicals; it is less condensed.the point at which spindle fibers attach during cell division; it is flanked by regions of heterochromatin. chromosomal regions that remain condensed at most times in all cells; for example, most of the y chromosome in humans.regions of chromosomes (or even whole chromosomes) that are heterochromatic in some cells and euchromatic in other cells of the same organism.a phenomenon that occurs when a gene is moved close to a heterochromatic region, such that its expression ceases.in mammals, the inactivated x chromosome in each cell that is completely heterochromatic and thus densely staining.this chemical modification to histone tails is associated with open chromatin