Other contributive mechanisms of tumor development involve blockade of the granzyme B/perforin pathway by overexpression of the serine protease inhibitor PI-9/SPI-6 [105], modifications in the antigen presentation system [106], developed resistance of tumor cells to apoptosis, and expression of indoleamine 2,3-dioxygenase (IDO) by the tumor or host stromal cells [107]. The gene discussed is PRF1; the disease is neoplasm.