We report testing of the specificity and utility of over 200 antibodies raised against 57 different histone modifications, in Drosophila melanogaster, Caenorhabditis elegans and human cells. While most antibodies performed well, over 25% failed specificity tests by dot blot or western blot. Among specific antibodies, over 20% failed in chromatin immunoprecipitation experiments. We advise rigorous testing of histone-modification antibodies before use and provide a website for posting new test results.

Variation in patterns of methylations of histone tails reflects and modulates chromatin structure and function1-3. To provide a framework for the analysis of chromatin function in C. elegans, we generated a genome-wide map of histone H3 tail methylations. We find that C. elegans genes show similarities in distributions of histone modifications to those of other organisms, with H3K4me3 near transcription start sites, H3K36me3 in the body of genes, and H3K9me3 enriched on silent genes. Unexpectedly, we also observe a striking novel pattern: exons are preferentially marked with H3K36me3 relative to introns. H3K36me3 exon marking is dependent on transcription and its level is lower in alternatively spliced exons, supporting a splicing related marking mechanism. We further show that the difference in H3K36me3 marking between exons and introns is evolutionarily conserved in human and mouse. We propose that H3K36me3 exon marking in chromatin provides a dynamic link between transcription and splicing.