Figure 1. (A) Canonical HP1 domain structure. The chromodomain (”chromo”) recognizes H3K9me, the hinge binds DNA and/or RNA, and the chromoshadow (”shadow”) homodimerizes and heterodimerizes. (B) Alternative paths underlying half-HP1 birth. Drift or selection drives the degeneration of the chromodomain (in this example) following a full HP1 duplication event. Alternatively, the duplication itself is restricted to a single domain. A 3′ retrotransposition bias⁴³ may underlie the enrichment of chromoshadow domain-only HP1s observed in our data set.¹