Heterochromatin protein 1 (HP1) is localized at heterochromatin sites, where it mediates gene silencing. A component of heterochromatin, HP1 recognizes and binds histone H3, leading to epigenetic repression. HP1 may interact with lamin B receptor (LBR), and this interaction can contribute to the association of the heterochromatin with the inner nuclear membrane. The Drosophila HP1 protein, product of the Su(var)2-5 locus, is a dose dependent regulator of gene silencing. HP1 interacts with compacted chromatin (heterochromatin) via its chromodomain (CD). The HP1 protein is also known to interact with a number of transcriptional regulators via its chromoshadow (CSD) domain, suggesting that HP1 is a bridging factor that couples particular chromatin structures with effects on gene expression.