Characterization of carbonic anhydrase IX (CA IX) as an endogenous marker of chronic hypoxia in live human tumor cells

CA IX is a stable marker of current or previous chronic hypoxia but influenced by nonhypoxic stimuli. Except the time course of accumulation, all properties of this marker resembled our previous findings for hypoxia-inducible factor-1α. A modified flow cytometry protocol provided good separability of CA IX-negative and -positive cells in vitro and can be applied to sort chronically hypoxic cells from tumors.