It is possible that somatic mutations induced by tobacco contribute to the formation of new renal cysts, by affecting the previously normal Pkd1 allele of tubular cells in Pkd1flox/-:Nestincre mice or the previously normal Pkd1 allele of Pkd1flox/flox:Nestincre tubular cells in which the Cre-lox system inactivated only one of the two copies, a phenomenon previously described for other targeted genes37,38. The gene discussed is PKD1; the disease is Renal cyst.