Carbonic anhydrase 9 is a HIF1α-regulated downstream gene, and is involved in the acidification of the extracellular matrix by hydrating carbon dioxide to produce bicarbonate and protons.9 In this study, we provide evidence that CA9 expression by cancer cells is directly related to intense infiltration of the tumour stroma by FOXP3+ Tregs in NSCLC. This evidence concerns the gene HIF1A and cancer.