Hypoxic tumours develop in an acidic microenvironment, owing to increased production of metabolic acids and poor vascularization.34 CA9 is highly induced in hypoxia, where it allows adaptation to this environment maintaining a more neutral intracellular pH.35, 36 Increased CA9 expression is a marker of poor prognosis in breast cancer and is more common in TNBC than other breast cancer subtypes.36 CA9 inhibition reduces tumour growth and metastasis.35, 36, 37. Here, CA9 is linked to breast cancer.