SERPINB9 and neoplasm: Our data suggests that PI9 overexpression by itself is not sufficient to impact killing by CAR T cells, but we recognize that in combination with other immunosuppressive factors such as hypoxia, soluble immunosuppressants (e.g., TGF-β), or PD-L1, and FasL expression on tumor cells can elevate the importance of PI9 as an escape mechanism [71–74].