We thank A. Berns for floxed p53 mice; the Princess Margaret Hospital/Ontario Cancer Institute Flow Cytometry Facility for cell sorting; the Campbell Family Institute for Cancer Research PCR Genotyping Facility for genotyping; and M. Saunders for scientific editing. This work was supported by grants to T.W. Mak and Z. Hao by the Canadian Institutes of Health Research. This research was also funded in part by the Ontario Ministry of Health and Long Term Care. The views expressed do not necessarily reflect those of the Ontario Ministry of Health and Long Term Care. The authors declare that they have no competing financial interests. Submitted: 4 July 2011 Accepted: 17 November 2011 REFERENCES Adhikary, S., F. Marinoni, A. Hock, E. Hulleman, N. Popov, R. Beier, S. Bernard, M. Quarto, M. Capra, S. Goettig, et al. 2005. The ubiquitin ligase HectH9 regulates transcriptional activation by Myc and is essen- tial for tumor cell proliferation. 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