The discovery of the involvement of tumour suppressor genes such as for example RB [58], p53 [59, 60, 61], PTEN [62, 63, 64] in glial neoplasms including glioblastomas, or genes involved in the sonic hedgehog signalling (SHH, PTCH, GLI, SUFU [65, 66]) or the wnt signalling pathway (beta‐catenin, [67], APC [68]) predominantly in medulloblastomas but also other CNS neoplasms, alongside with the discovery and characterisation of some of these genes in Drosophila prompted the generation of deletion mutants in mice, before the Cre‐lox technology was either established or more widely adopted. Here, PTEN is linked to medulloblastoma.