Gap genes stand at the top of the zygotic segmentation hierarchy in Drosophila. Their expression domains are set up by a combination of maternal regulatory signals and interactions among themselves. In addition, these regulatory pathways are partially redundantly specified by the maternal and zygotic genomes, or by multiple zygotic gene products. The gap genes code for transcription factors which exert their function by forming short-range morphogenetic gradients; differential concentrations of these transcription factors can either act as activators or as repressors for the expression domains of neighboring genes. It is possible to view the gap genes as a system of genes that subdivides the embryo by forming an activation-repression cascade proceeding from anterior towards posterior.