Confocal microscopy showed that a high intracellular acidity characterises prostate cancer cells stably cultured at pH 6.5, as demonstrated by the massive intake of BCECF in these cells; this was consistent with a clear co-localization of the exosomal marker CD9 and CA IX, mainly at the plasma membrane, while co-expressed at the cytoplasm and the nuclear membrane levels, as well. Here, CA9 is linked to prostate cancer.