Biomolecular condensates can be regulated by modifying the underlying multivalent interactions. (A) Reversible covalent modifications of chromatin. Nucleosomal histones can be reversibly modified, leading to changes in chromatin state. A model is depicted where chromatin alone can form a condensate (center, gray) mediated by internucleosomal contacts (dotted lines). Methylation of histones at histone H3K9 can recruit HP1α via chromodomain binding and produce a condensate rich in HP1α and other heterochromatin factors (left). In contrast, acetylation of histones at multiple lysine residues can reduce internucleosomal interactions, exposing TF-binding sites on DNA and recruit bromodomain-containing factors, leading to a condensate rich in components of the transcriptional machinery (right). B) Local RNA synthesis. A nascent transcript tethered to the elongating polymerase can be bound by many RNA processing enzymes, leading to a condensate rich in RNA processing machinery. Abbreviations: HP1α, heterochromatin protein 1α; TF, transcription factor.