The best characterized function of Bmi1 is the control of proliferation of undifferentiated progenitor cells mainly through repression of the Ink4a/Arf tumour suppressor gene locus [32,33], which in turn regulates the activity of cyclin D, Cdk4/Cdk6 and p53 (reviewed in [34]). The gene discussed is TP53; the disease is neoplasm.