HP1c casts light on dark matter.

Heterochromatin protein 1a (HP1a) is well known as a silencing protein and regulates gene expression through its binding to methylated histone H3K9. Despite sharing a nearly identical domain architecture, most eukaryotes have at least three HP1 homologs that have differential localization patterns and functions in heterochromatin and euchromatin. Among the three main HP1 paralogs in Drosophila, HP1c functions the least like the canonical heterochromatic HP1, HP1a. HP1c exclusively localizes to euchromatin. In contrast to a key role of HP1a in heterochromatin formation and gene silencing, recent studies link transcriptional elongation by RNA polymerase II to euchromatic HP1c or HP1γ. These findings expand the role for HP1c in targeting euchromatin, suggesting that HP1c acts as a positive regulator in active transcription in euchromatin. Here, we highlight recent data on the specificity and function of HP1c.