CSTB and neoplasm: However, whereas no change in the proliferation of PyMT tumor cells in vivo and in vitro was detected in StfB-/- PyMT mice, a significant decrease in the Ki67 and BrdU proliferation index was measured in CstC knock-out PyMT primary tumors and isolated PyMT cells, indicating a potential role for cystatin C in the regulation of the proliferative activity of tumor cells.