In this study, to clarify the role of IGF-I in physiological (development) and pathological (tumorigenesis) settings in the cerebellum, we cross-bred transgenic mice, overexpressing IGF-I (IGF-I Tg) in neural progenitors under control of regulatory sequences from the nestin gene [15], with Ptc1+/- mice, a faithful model of human medulloblastoma. The gene discussed is NES; the disease is medulloblastoma.