Working model of histone modification profiles at imprinted genes undergoing developmental repression. In mouse embryonic stem cells, the presence of a germline differentially methylated region reflecting an imprinting control region at the promoter of gene A confers a distinct histone modification profile to that of gene B, which has no differential DNA methylation; gene A is marked by H3K4me3 on the active allele and by H3K9me3 plus H4K20me3 on the inactive allele. After developmental repression, both genes acquire H3K27me3 on the previously active allele. Transcriptional activators and repressors are key players in this process and may be a cause or consequence of the histone modification states shown. All histone modifications illustrated represent the trimethylated state.