VHL is the substrate binding protein of a CUL2-based E3 ubiquitin ligase complex that conjugates ubiquitin to hydroxylated HIF-alpha (Iwai et al. 1999, Kamura et al. 2000, Ohh et al. 2000, Groulx and Lee 2002, Maynard et al. 2003). VHL is predominantly cytosolic and shuttles between the cytosol and the nucleus (Lee et al. 1999, Groulx and Lee 2002). Ubiquitination and degradation of HIF-alpha can occur in both the cytosol and the nucleus (Berra et al. 2001). Upon return to normoxia from hypoxia most ubiquitinated HIF-alpha is nuclear (Groulx and Lee 2002).