H3 lysine 9 trimethylation (H3K9me3) is a histone posttranslational modification 
(PTM) that has emerged as hallmark of pericentromeric heterochromatin. This 
constitutive chromatin domain is composed of repetitive DNA elements, whose 
transcription is differentially regulated. Mammalian cells contain three HP1 
proteins, HP1α, HP1β and HP1γ These have been shown to bind to H3K9me3 and are 
thought to mediate the effects of this histone PTM. However, the mechanisms of 
HP1 chromatin regulation and the exact functional role at pericentromeric 
heterochromatin are still unclear. Here, we identify activity-dependent 
neuroprotective protein (ADNP) as an H3K9me3 associated factor. We show that 
ADNP does not bind H3K9me3 directly, but that interaction is mediated by all 
three HP1 isoforms in vitro. However, in cells ADNP localization to areas of 
pericentromeric heterochromatin is only dependent on HP1α and HP1β. Besides a 
PGVLL sequence patch we uncovered an ARKS motif within the ADNP homeodomain 
involved in HP1 dependent H3K9me3 association and localization to 
pericentromeric heterochromatin. While knockdown of ADNP had no effect on HP1 
distribution and heterochromatic histone and DNA modifications, we found ADNP 
silencing major satellite repeats. Our results identify a novel factor in the 
translation of H3K9me3 at pericentromeric heterochromatin that regulates 
transcription.