In the developing mouse neural tube, Sonic Hedgehog (Shh) is initially expressed in the notochord (NC) underlying the ventral neural tube; as development progresses, Shh auto-induces a secondary domain of Shh production within the floor plate (FP)  of the neural tube at the ventral midline.  Several lines of evidence indicate that Shh acts in a concentration dependent manner to specify all ventral cell types of…