CTSZ and breast cancer: Therefore, we performed a quantitative secretome comparison using differential isotope labeling of the cell-conditioned medium (CCM) of co-cultures comprising PyMT breast cancer cells and Mɸ either wild type or double knock-out for CTSB and CTSZ (Mɸ Ctsb−/−; Ctsz−/−) (Fig. 1a).