VHL and infection: Western blotting 4 days after infection confirmed the high efficiency of gene deletion but after approximately 20–30 days of continuous proliferation, a significant restoration of pVHL expression was evident in the Vhlfl/flTrp53fl/fl Cre-infected cell population (Fig 1F), suggesting a mixed cell population in which Vhl null cells have a proliferative disadvantage in comparison to an initially rare population of pVHL-expressing cells (see results below).