Abstract
Heterochromatin spreading, the propagation of repressive chromatin along the chromosome, is a reaction critical to genome stability and defense, as well as maintenance of unique cell fates. Here, we discuss the intrinsic properties of the spreading reaction and circumstances under which its products, formed distal to DNA-encoded nucleation sites, can be epigenetically maintained. Finally, we speculate that the epigenetic properties of heterochromatin evolved together with the need to stabilize cellular identity.