CA9 and cancer: In contrast, CAIX is an endogenoushypoxia marker that, due to its long half-life (38 h), is better suitedto visualize chronic hypoxia.24,25 CAIX-expressing cancercells represent a population adapted to chronic hypoxia and are, therefore,highly relevant in the context of cancer immunology research.26 Therefore, we pose CAIX as a suitable targetfor functional imaging of cancer cells that have adjusted to chronichypoxic conditions.