Fig. 1. In the developing Drosophila VNC, a small group of 90 neurons express the ap gene. Most ap-neurons extend axons in a common fascicle running the length of the VNC. The Tv neurons are unique; they innervate the DNH and express the Fmrf neuropeptide. Specification of Tv neurons is dependent upon a combinatorial code of ap, sqz and a target-derived BMP signal mediated by the Gbb ligand and the Wit receptor (Allan et al., 2003; Marques et al., 2003).