In several studies, CAIX expression appears closely associated with tumor hypoxia (Jankovic et al. 2006; Loncaster et al. 2001; Airley et al. 2003), while in others the relationship between CAIX expression and hypoxia is unclear (Hedley et al. 2003; Mayer et al. 2005). This evidence concerns the gene CA9 and neoplasm.