- The human ubiquitin-conjugating enzyme Cdc34 controls cellular proliferation through regulation of p27Kip1 protein levels. Butz, N., Ruetz, S., Natt, F., Hall, J., Weiler, J., Mestan, J., Ducarre, M., Grossenbacher, R., Hauser, P., Kempf, D., Hofmann, F. Exp. Cell Res. (2005)