The PHD finger is a common structural motif found in all eukaryotic genomes. It is a Zn(2+)-binding domain and its closest structural relative is the RING domain. Many RING fingers bind to E2 ligases to mediate the ubiquitination of proteins. Whether PHD fingers share a common function is unclear. Notably, many if not all PHD fingers are found in nuclear proteins whose substrate tends to be chromatin. Some PHD fingers bind to specific nuclear protein partners, apparently through the same surface that is used by RING domains to bind their cognate E2 ligases. New evidence also suggests that some PHD fingers bind to nucleosomes, raising the possibility that chromatin might be a common nuclear ligand of PHD fingers.