After we assessed acquired tamoxifen resistance in 235 breast cancer patients and a list of breast cancer cell lines, we developed poly(lactic‐co‐glycolic acid)‐poly(ethylene glycol)‐carbonic anhydrase IX mono antibody nanobubbles (PLGA‐PEG‐mAbCAIX NBs) to detect hypoxic breast cancer cells upon exposure of tamoxifen in nude mice. Here, CA9 is linked to breast cancer.