Considering the importance of OPCs in the etiology of various demyelinating diseases and the role of PDGF-AA and Shh on OPC proliferation and recruitment to demyelinating lesions [8], further work should address whether Shh signaling at the PC of oligodendrocyte precursors is implicated in cell proliferation after external insults in vivo and/ or as part of normal homeostasis. Here, SHH is linked to demyelinating disease.