PTCH1 and neoplasm: The increase of proliferation observed in Ptch1+/−/Tis21KO vs. Ptch1+/−/Tis21WT tumors is not present in either GCPs or MB lesions of 2-, 6- or 12-week-old mice (27), indicating that this proliferative phenotype is tumor-specific; conversely, in Ptch1+/−/Tis21KO cerebella during either the physiological or preneoplastic development the arrest of migration of GCPs prevails (27).