Fig. 8. Possible evolution of the Dll and Dlx genes and their functions. Modified from Panganiban et al. (Panganiban et al., 1997). The ancestral Dll gene may have functioned first in the developing nervous system, acquiring roles in appendage development later in evolution. Dll was duplicated multiple times in the deuterostome lineage to give rise to the present day six Dlx genes in mice and humans and the eight Dlx genes in zebrafish. See text for details. Not shown is the acquisition of Dll/Dlx functions in other tissues, including the branchial arches, the otic and olfactory systems, and hematopoietic system. It is not yet known when these roles were acquired or whether they predate the divergence of the protostomes and deuterostomes.