The Polycomb group (PcG) genes are essential for maintaining homeotic gene repression during development. Mutations in these genes can be suppressed by mutations in SWI/SNF family genes. A complex called PRC1, containing products of PcG genes such as Polycomb, Posterior sex combs, polyhomeotic, and Sex combs on midleg, was purified. PRC1 blocks the ability of nucleosomal arrays to be remodeled by SWI/SNF, suggesting competition between PRC1 and SWI/SNF for the nucleosomal template. PRC1 interacts with histone tails and is active on nucleosomal arrays formed with tailless histones. PRC1 and SWI/SNF complexes may compete for the same nucleosomal template. PRC1 stabilizes chromatin against remodeling by SWI/SNF, and this inhibition does not require ATP. PRC1 must bind to the template first to inhibit SWI/SNF remodeling. PRC1 does not directly inactivate SWI/SNF, as preincubation of PRC1 with SWI/SNF does not inactivate remodeling. PRC1 functions by interacting with nucleosomal arrays to stabilize them against subsequent remodeling. PRC1 and SWI/SNF can block each other's function. PRC1 blocks SWI/SNF function only if it is added to nucleosomal arrays before SWI/SNF. PRC1 does not require histone tails for its function, distinguishing it from repressive complexes like deacetylase complexes. PRC1 contains proteins such as PC, PSC, PH-p, PH-d, and SCM, but not PCL and E(Z). PRC1 and SWI/SNF complexes may interact with identical or overlapping faces on the nucleosome, leading to competition. PRC1 may stabilize nucleosomal DNA by multiple binding sites or through looping. PRC1 and SWI/SNF complexes are conserved from flies to mammals and may have conserved interactions. PRC1 helps maintain a repressed state by blocking chromatin remodeling by SWI/SNF. PRC1 is involved in creating the repressed state and may require other complexes for targeting. PRC1 and SWI/SNF complexes may have similar mechanisms of interacting with nucleosomes. PRC1 is a complex containing multiple PcG proteins, and there are likely other PcG complexes in addition to PRC1. PRC1 and SWI/SNF complexes may compete for the same nucleosomal template, and PRC1 must bind to the template first to inhibit SWI/SNF remodeling. PRC1 functions by interacting with nucleosomal arrays to stabilize them against subsequent remodeling. PRC1 and SWI/SNF complexes may have conserved interactions, and PRC1 may help maintain a repressed state by blocking chromatin remodeling by SWI/SNF. PRC1 is a complex containing multiple PcG proteins, and there are likely other PcThe Polycomb group (PcG) genes are essential for maintaining homeotic gene repression during development. Mutations in these genes can be suppressed by mutations in SWI/SNF family genes. A complex called PRC1, containing products of PcG genes such as Polycomb, Posterior sex combs, polyhomeotic, and Sex combs on midleg, was purified. PRC1 blocks the ability of nucleosomal arrays to be remodeled by SWI/SNF, suggesting competition between PRC1 and SWI/SNF for the nucleosomal template. PRC1 interacts with histone tails and is active on nucleosomal arrays formed with tailless histones. PRC1 and SWI/SNF complexes may compete for the same nucleosomal template. PRC1 stabilizes chromatin against remodeling by SWI/SNF, and this inhibition does not require ATP. PRC1 must bind to the template first to inhibit SWI/SNF remodeling. PRC1 does not directly inactivate SWI/SNF, as preincubation of PRC1 with SWI/SNF does not inactivate remodeling. PRC1 functions by interacting with nucleosomal arrays to stabilize them against subsequent remodeling. PRC1 and SWI/SNF can block each other's function. PRC1 blocks SWI/SNF function only if it is added to nucleosomal arrays before SWI/SNF. PRC1 does not require histone tails for its function, distinguishing it from repressive complexes like deacetylase complexes. PRC1 contains proteins such as PC, PSC, PH-p, PH-d, and SCM, but not PCL and E(Z). PRC1 and SWI/SNF complexes may interact with identical or overlapping faces on the nucleosome, leading to competition. PRC1 may stabilize nucleosomal DNA by multiple binding sites or through looping. PRC1 and SWI/SNF complexes are conserved from flies to mammals and may have conserved interactions. PRC1 helps maintain a repressed state by blocking chromatin remodeling by SWI/SNF. PRC1 is involved in creating the repressed state and may require other complexes for targeting. PRC1 and SWI/SNF complexes may have similar mechanisms of interacting with nucleosomes. PRC1 is a complex containing multiple PcG proteins, and there are likely other PcG complexes in addition to PRC1. PRC1 and SWI/SNF complexes may compete for the same nucleosomal template, and PRC1 must bind to the template first to inhibit SWI/SNF remodeling. PRC1 functions by interacting with nucleosomal arrays to stabilize them against subsequent remodeling. PRC1 and SWI/SNF complexes may have conserved interactions, and PRC1 may help maintain a repressed state by blocking chromatin remodeling by SWI/SNF. PRC1 is a complex containing multiple PcG proteins, and there are likely other Pc