Chromatin priming of genes in development: Concepts, mechanisms and consequences

•Chromatin of developmentally regulated genes is primed before gene activation.•Chromatin priming is required for the correct temporal regulation of gene expression.•Chromatin priming maintains a memory of previous gene activation.

During ontogeny, cells progress through multiple alternate differentiation states by activating distinct gene regulatory networks. In this review, we highlight the important role of chromatin priming in facilitating gene activation during lineage specification and in maintaining an epigenetic memory of previous gene activation. We show that chromatin priming is part of a hugely diverse repertoire of regulatory mechanisms that genes use to ensure that they are expressed at the correct time, in the correct cell type, and at the correct level, but also that they react to signals. We also emphasize how increasing our knowledge of these principles could inform our understanding of developmental failure and disease.