CTSS and central nervous system cancer: Growing evidence suggests that microglia and macrophages provide the main source of tumor-promoting molecules, such as matrix metalloproteinases, cathepsins and other endopeptidases that are involved in extracellular matrix degradation and are considered to be the key enzymes involved in glioma invasion [7, 8, 45, 48, 49, 50, 51, 52].