Robo-1/2, and Robo-3, commissural axons are guided toward the midline to cross the neural tube and prevented from crossing back. The signaling molecules guiding the growth and projections of commissural neurons have well studied homologs in invertebrates. In the Netrin/DCC chemoattraction pathway the C. elegans homologs are Unc-6/Unc-40 and Unc-5 while the Drosophila homologs are Netrin-A and Netrin-B/Frazzled and Dunc5. In the Slit/Robo chemoreppelant pathway the C. elegans homologs are Slt-1/Sax-3 whereas the Drosophila homologs are also known as Slit/Robo(1-3). Glial fate mapping In the central nervous system (CNS), overall cell fate mapping is typically directed by the sonic hedgehog (Shh) morphogen signaling pathway. In the spinal cord, Shh is directed by both the notochord and floor plate regions which ultimately drives the organization of neural and glial progenitor populations. The specific glial populations impacted by Shh in these two regions include oligodendrocyte precursor cells (OPCs), oligodendrocytes, NG2+ cells, microglia, and astrocytes. The floor plate (FP) region of the spinal cord individually contributes to gliogenesis, or the formation of glial cells. Traditionally, progenitor cells are driven from their progenitor expansion phase, to the neurogenic phase, and ultimately to the gliogenic phase. From the gliogenic phase, the former progenitor cells can then become astrocytes, oligodendrocytes, or other more specialized glial cell types. Recently, there have been efforts to use conditional mutagenesis to selectively inactivate the Shh pathway specifically in the FP region to identify different roles of molecules involved in oligodendrocyte cell fate. Oligodendrocytes are the cells responsible for myelinating axons in the CNS. Shh regulates Gli processing through two proteins, Ptch1 and Smo. When Shh is not active, Ptch1 is responsible for suppressing the pathway through the inhibition of Smo. Smo is crucial to the overall transduction of signal of the Shh pathway. If Smo is inhibited, the Shh pathway is also inactive, which ultimately represses gliogenesis. Specific factors such as Gli3 are required for oligodendrocyte cell fate. Since Shh regulates Gli processing, if Smo is compromised or inhibited by Ptch1, this inactivates the Shh pathway and prevents Gli processing which disrupts glial cell fate mapping. Shh signaling in the FP region is very important because it needs to be active in order for gliogenesis to occur. If Shh is inactivated within the FP region and activated in other regions of the spinal cord such as the Dbx or pMN domains, gliogenesis is compromised. But, when Shh is active in