GZMB and neoplasm: Beyond this, tumor cells express constitutively high endogenous levels of serine protease inhibitors (SERPINS) such as serine protease inhibitor 9, (PI-9) which inhibits proteolytic activity of granzyme B and is associated with poor outcome and response to immunotherapy in melanoma (176–179); importantly, expression of PI-9 has been shown to increase in tumor cells in response to IFNγ, increasing the challenge posed during CTL attack (180).