The vast diversity of cell types in the nervous system arises from the activity of morphogens during development. Morphogens are soluble, long-range signaling molecules that specify diverse cell fates in a concentration-dependent manner. At the ventral region of the spinal cord, the morphogen sonic hedgehog (Shh) generates four interneuron progenitor domains, one motoneuron progenitor domain and the Floor Plate. Shh signaling initiates after binding to the membrane receptor Ptch1, an event leading to activation of Gli transcription factors. The evidence suggests that graded Shh signaling is transformed into a gradient of Gli activity that organizes the ventral region1. Yet, the molecular mechanisms of neuronal specification by Shh are not well understood. An ongoing mutagenesis screen of enhancer regions will provide further insights about the identity of transcription factors that cooperate with Gli proteins during interpretation of the Shh gradient in the neural tube.