In Nature this week, an international research team reports their examination of the alterations in chromatin structure that coincide with pluripotency and genome activation. Using a zebrafish model, the team mapped the location of the repressive H3K27me3 and the activating H3k4me3 histone molecules. While neither were detected before the maternal-zygotic transition, more than 80 percent of the genes were marked by H3K4me3 after genome activation.