This is the staining phenomenon seen in the chromosomes with reference to heterochromatic and euchromatic regions. The heterochromatic regions which stain deeply are said to exhibit heteropycnosis as against euchromatin which stains lightly. However, it should be noted that the heterochromatin always does not stained deeply with reference to euchromatin. While the euchromatic region is always constant in its behaviour (non heteropycnotic) towards staining (has constant coils), the heterochromatic region may stain deeply or lightly and this is known as differential heteropycnosis.