Transcriptional silencing by Polycomb group (PcG) proteins, which is crucial during development, requires histone H3 lysine 27 trimethylation(H3K27me3). The Trithorax protein (TRX) counteracts PcG silencing, but what underpins this antagonism? Tie, Harte and co-workers now propose that, in Drosophila, H3K27 acetylation (H3K27ac) by the TRX-associated histone acetyltransferase CBP might be involved (see p. 3131). They show that CBP acetylates H3K27 (which requires TRX), and that CBP or TRX overexpression increases H3K27ac levels while decreasing H3K27me3 levels and causing PcG silencing defects. Similarly, RNAi-mediated knockdown of the PcG protein E(Z) decreases H3K27me3 and increases H3K27ac. These data suggest that H3K27 acetylation and trimethylation are alternative modifications at the same site. In support of this, the high H3K27ac levels found in early embryos decline as H3K27me3 levels increase, and genome-wide ChIP-chip analysis reveals that H3K27me3 and H3K27ac are mutually exclusive at PcG target genes. From their results, the authors propose that TRX-dependent H3K27 acetylation by CBP prevents H3K27 trimethylation, thereby antagonising PcG silencing.