SHH and central nervous system cancer: Primary cilia are microtubule-based organelles which can mediate signaling that influences the proliferation of normal and tumor cells [e.g., through the mammalian Sonic Hedgehog (SHH), platelet-derived growth factor (PDGF), and Notch pathways], including various cancers of the CNS (medulloblastoma, ependymoma, choroid plexus tumors) [6–8].