Sonic hedgehog is the secreted protein which mediates signaling activities of the notochord and floor plate. Studies involving ectopic expression of SHH in vitro and in vivo result in floor plate induction, and differentiation of motor neuron and ventral interneurons. On the other hand, mice mutant for SHH lack ventral spinal cord characteristics. In vitro blocking of SHH signaling using antibodies against it shows similar phenotypes. SHH exerts its effects in a concentration-dependent manner, so that a high concentration of SHH results in a local inhibition of cellular proliferation. This inhibition causes the floor plate to become thin compared to the lateral regions of the neural tube. Lower concentration of SHH results in cellular proliferation and induction of various ventral neural cell types. Once the floor plate is established, cells residing in this region will subsequently express SHH themselves generating a concentration gradient within the neural tube.