About 30% of medulloblastomas (MBs), the pediatric tumor of the cerebellum, originates from the granule neuron precursor cells (GCPs) located in the external granular layer (EGL), at the surface of the developing cerebellum, in consequence of hyperactivation of the Sonic Hedgehog (Shh) pathway (Kadin et al., 1970; Schüller et al., 2008; Yang et al., 2008; Gibson et al., 2010; Northcott et al., 2012). The gene discussed is SHH; the disease is medulloblastoma.