Cis-regulatory modules of metazoan genomes determine the when and where of gene expression during development. Here we discuss insights into the genetic and molecular mechanisms behind cis-regulatory module usage that have come from recent application of genomics assays to insect metamorphosis. Assays including FAIRE-seq, ATAC-seq, and CUT&RUN indicate that sequential changes in chromatin accessibility play a key role in mediating stage-specific cis-regulatory module activity and gene expression. We review the current understanding of what controls precisely coordinated changes in chromatin accessibility during metamorphosis and describe evidence that points to systemic hormone signaling as a primary signal to trigger genome-wide shifts in accessibility patterns and cis-regulatory module usage
