Cellular memory of transcriptional states by Polycomb-group proteins.

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