Polycomb-group proteins

Polycomb-group proteins are a family of proteins first discovered in fruit flies that can remodel chromatin such that transcription factors cannot bind to promoter sequences in DNA. Polycomb group proteins play a role in silencing HOX genes through modulation of chromatin structure.

In insects
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. Polycomb Gene complexes or PcG silencing involves at least three kinds of multiprotein complex PRC1, PRC2 and PhoRC which work together to carry out their repressive effect.

In mammals
In humans Polycomb Group gene expression is important in many aspects of development. Although pathogenic mutations in polycomb group genes have not been identified, abnormal levels of PcG proteins correlate with the severity and invasiveness of several cancer types. The mammalian PRC1 core complexes are very similar to Drosophila.