GDNF and Parkinson disease: In particular, the knockdown of SHH alone or in combination with GDNF caused a profound reduction in NPC-mediated neuroprotection noted both the lateral tier of substantia nigra, which houses the PD relevant A9 dopamine neurons, and the striatum (Fig 2I–2L), when compared to the control (Fig 2E and 2F) and silencing GDNF alone (Fig 2G and 2H).