MedulloblastomaCerebellar NeoplasmsNeuroectodermal Tumors, PrimitiveHedgehog ProteinsBrain NeoplasmsCranial IrradiationInfratentorial NeoplasmsCerebellumDisease Models, AnimalAnaplasiaCell Line, TumorEpendymomaMice, TransgenicBasal Cell Nevus SyndromeVeratrum AlkaloidsCranial Fossa, PosteriorCell ProliferationGene Expression Regulation, NeoplasticChromosomes, Human, Pair 17Signal TransductionMice, NudeCombined Modality TherapyImmunohistochemistry