The majority of mice with homozygous deletion of such tumour suppressor genes showed severe phenotypes, often with embryonic lethality, whilst the heterozygous mutants develop neoplasms including in the CNS, for example pituitary tumours in Rb‐heterozygous mice [69, 70], medulloblastomas in Ptch [71] or Sufu [72] heterozygous mice. The gene discussed is PTCH1; the disease is medulloblastoma.