Mouse Spi-6, also known as serpin B9, is a cytoplasmic serine protease inhibitor of ~42kD. Spi-6 is expressed in dendritic cells, lymphocytes and specific cell types at immune- privileged sites. It inactivates granzyme B, a natural occurring inducer of programmed cell death. As such, Spi-6 functions as anti-apoptotic protein. In cytotxic T cells (CTL), Spi-6 is required to protect clonal bursts (expansion) from granzyme B induced cell death. However, it is not required for the development of memory cells. Expression of SPI-6 is of crucial importance in the escape of several tumors from a CTL response and is, therefore, a parameter that influences the feasibility of CTL-mediated immunotherapy of cancer.