The DistaL-Less (DLL) gene directs cells to create organs that bud off from a main body axis, including such appendages as legs and antennae. For example, developmental biologists have long known that DLL controls the development of the distal portion of insect limbs, that is, the portion farthest from the body. The gene encodes a homeodomain protein, one of a family of transcription factors that, among other functions, act to define body axes in vertebrates and invertebrates. The DLL protein in turn activates other genes that govern limb formation. However, the pathway by which DLL controls downstream genes remains unknown.

DLL itself is turned on by the two genetic pathways that control formation of the dorso-ventral and anterior-posterior axes. The gene's expression also is governed by homeotic proteins. For example, in Drosophila, DLL expression for legs and wings occurs in the thoracic limb-forming disks, but expression in the abdomen is repressed by homeotic proteins. As a result, these segments lack appendages.

Developmental biologists have known for a long time about the role of DLL in insect limbs. However, it has recently been discovered that the gene occurs broadly in other animals, initiating the development of appendages in crustaceans. It has been shown that DLL is more pervasive within the animal kingdom than anyone has previously suggested. This finding yields new understanding about the origin of appendages and about the Pre-Cambrian precursor to modern phyla.