The introduction of oncogenic mutations into this cell population has been used to model brain tumours, for example by Cre-mediated recombination of tumour suppressor genes such as Pten, p53, Rb or Nf1 (Chow et al., 2011; Jacques et al., 2010; Zheng et al., 2008; Zhu et al., 2005). The gene discussed is PTEN; the disease is brain neoplasm.