Polycomb-group proteins
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A family of proteins first discovered in fruit flies that can remodel chromatin such that transcription factors cannot bind to promoter sequences in DNA. In Drosophila, the Trithorax-group (trxG) and Polycomb-group (PcG) proteins act antagonistically and interact with chromosomal elements, termed Cellular Memory Modules (CMMs). Trithorax-group (trxG) proteins maintain the active state of gene expression while the Polycomb-group (PcG) proteins counteract this activation with a repressive function that is stable over many cell generations and can only be overcome by germline differentiation processes.
In humans Polycomb Group gene expression is important in many aspects of development. Mutations in polycomb group genes have been associated with several types of cancers, and abnormal levels of several PcG proteins correlate with the severity and invasiveness of certain types of cancer.
