Identification of the TMPRSS2–ETS fusion gene in approximately 20% of PIN lesions suggests that it is an early event in prostate tumourigenesis 71 and our recent findings that TMPRSS2–ERG is expressed in α2β1hi/CD133+ cells from prostate tumours 48 supports the hypothesis that the cell of origin of prostate cancer is a stem cell. This evidence concerns the gene ERG and prostate carcinoma.