CA9 and breast cancer: Previous studies have demonstrated that overexpression of hypoxia-associated proteins at diminished oxygen levels, including HIF-1α and its downstream target, carbonic anhydrase IX (CAIX), is associated with suppressed oestrogen receptor-α (ER-α) levels [5, 6], a dedifferentiated phenotype [7], resistance to endocrine treatment [8–10], breast cancer recurrence [11], and shorter disease-free survival [12].