To gain further evidence for an epithelial origin of our engineered tumours we next genetically marked renal epithelial cells in vivo, labelling all nephron segments by doxycycline feeding of mice harbouring transgenes encoding Pax8 promoter-regulated expression of rtTA, Tet-responsive Cre expression33 and ROSA26-lox-STOP-lox-regulated tdTomato expression34(Pax8-rtTA;TRE-LC1;R26R-LSL-tdTomato) (Fig. 4a). The gene discussed is MAP6; the disease is neoplasm.