Four MAbs, three of them A549-positive (clones A3, F4, and H7) and one clone A549-negative (E3), were further selected to analyze CA IX expression by flow cytometry in different human tumor cells (cell lines A549, U-87, A498, MCF-7, A431, CaSki, HeLa, and Jurkat) grown under normoxic and hypoxic conditions. This evidence concerns the gene CA9 and neoplasm.