Heterochromatin plays an essential role in nuclear organization and regulation of gene expression by directing the 3D genome organization, regulating lineages-specific gene expression and ensuring repression of transposable elements and endogenous retroviruses. Functionally and structurally different chromatin domains are discriminated by the so-called histone code, combinations of post-translational modifications of histones that are deposited by code-writers and recognized by code-readers. The main mark of heterochromatin, trimethylation of histone H3 tail at lysine 9 (H3K9me3) is deposited by histone methyltransferases, such SetDB1, and provides a binding platform for readers, most significantly, HP1 family proteins. How heterochromatin spreads from nucleation sites to establish large repressive domains and how these domains are stably maintained is not fully understood. Using a reporter to monitor dynamics of heterochromatin establishment and maintenance, we show the existence of a feedback mechanism by which the reader of the H3K9me3 mark, HP1, attracts the writer, SetDB1 through direct physical interaction that depends on posttranslational modifications of SetDB1. These modifications of SetDB1 are required for the spreading and stable maintenance of heterochromatin