Test graduate-level recall of canonical histone post-translational modifications—specifically H3K4me3, H3K27ac, H3K27me3, and H3K9me3—including typical genomic localization (promoters, enhancers, heterochromatin), their effects on transcriptional activity (activation vs repression), and common writer/eraser/reader families. Scope: identification and immediate functional consequences; no experimental design or data analysis required.