AbstractIn development, cells pass on established gene expression patterns to daughter cells over multiple rounds of cell division. The cellular memory of the gene expression state is termed maintenance, and the proteins required for this process are termed maintenance proteins. The best characterized are proteins of the Polycomb and trithorax Groups that are required for silencing and maintenance of activation of target loci, respectively. These proteins act through DNA elements termed maintenance elements. Here, we re‐examine the genetics and molecular biology of maintenance proteins. We discuss molecular models for the maintenance of activation and silencing, and the establishment of epigenetic marks, and suggest that maintenance proteins may play a role in propagating the mark through DNA synthesis. Developmental Dynamics 232:633–655, 2005. © 2005 Wiley‐Liss, Inc.