We demonstrated thatthe developed B9 tracers have a moderate affinityto CAIX in vitro and that they target head and neckcancer xenografts in vivo. This targeting was entirelyCAIX specific for native B9, and although the addition of ABDs increasedplasma residence and tumor uptake, tumor uptake of the ABD-modifiedVHH was partly CAIX-independent in keratinized areas. This evidence concerns the gene CA9 and neoplasm.