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 the developing neural tube (PV3, PMN, PV2, PV1, PV0).  Importantly, markers exist for these cell types (center panel), allowing for both qualitative and quantitative assesment of alterations in Shh signaling during neural patterning.  A representative example of such marker analysis is shown in the right panel.  Yellow, FoxA2; purple, Nkx2.2; green, Olig2; Blue, Nkx6.1; Red, Pax7.