HP1α (heterochromatin protein 1 alpha) is a structural chromosomal protein that plays a multifaceted role in chromatin regulation and gene expression. It is primarily associated with heterochromatin, a condensed form of chromatin that promotes gene silencing, particularly at telomeres and centromeres. HP1α binds to the histone modification H3K9me3 (histone H3 lysine 9 trimethylation) via its N-terminal chromodomain, which is crucial for the assembly and maintenance of heterochromatin. This binding helps to compact chromatin, control gene expression, and maintain genome integrity. In addition to its role in gene silencing, HP1α also has the capacity to promote gene expression. It can impact the expression of genes in euchromatin as well as heterochromatic genes. The mechanisms by which HP1α can positively regulate gene expression include maintenance of heterochromatin structure and facilitating transcriptional elongation. HP1α is also involved in the induction of heat-shock genes and has been shown to interact directly with RNA polymerase II and heterogeneous nuclear ribonuclear proteins (hnRNPs), suggesting a role in RNA processing.