CA9 and breast carcinoma: When stratified by subtype, the high CAIX group was clearly associated with shorter DFS (HR = 2.09, 95% CI =1.11-3.92, <i>p</i> = 0.02) and OS (HR = 2.50, 95% CI =1.53-4.07, <i>p</i> = 0.0002) in TNBC and shorter DFS in ER<sup>+</sup> breast cancer (HR = 1.81 95% CI =1.38-2.36, <i>p</i> < 0.0001).<h4>Conclusion</h4>High CAIX expression is a negative prognostic marker of breast cancer regardless of the subtypes.