Heterochromatin spreading is the process of condensing DNA in a wave-like pattern, along its length. This allows an autocatalytic process to occur, forming heterochromatin in linear pattern from a focus.
Gene insulators are regulatory proteins, restricting the spread of heterochromatin into actively transcribed regions, as well as regulatory elements (silencers, enhancers, …). By being able to define the boundaries of actively transcribed genes, it is possible to have heterochromatic and euchromatic regions on a single chromosome.