The inheritable methylation pattern of gene activity, created upon cell differentiation, is further maintained by the "SET" (methyltransferase)-domain proteins. However, it is still not clear how SET-proteins can decide on the required gene activity state and the way their chromatin association is maintained. Here we have found that high levels of histone acetylation - the hallmarks of active chromosome regions in vivo - can increase the affinity of reconstituted nucleosomes to the SET domain of ALL-1 histone methyltransferase in a defined system in vitro. (C) 2010 Elsevier Inc. All rights reserved.