Cellular memory of transcriptional states by Polycomb-group proteins

The Polycomb-group constitutes an important, widely conserved group of tran scriptional repressors best known for their function in stably maintaining the inactive expression patterns of hey developmental regulators, including homeotic genes. Together with the counteracting trithorax-group proteins, they establish a form of cellular memory by faithfully maintaining transcri ption states determined early in embryogenesis. Besides being crucial for t he correct execution of developmental programs, Polycomb-group mediated sil encing also appears to be involved in controlling cell proliferation. Here we discuss several aspects of Pc-G function: target gene specificity and re cognition as well as propagation of inactive chromatin states to subsequent cell generations.