A separate study in MCF-7 breast cancer cells demonstrated that E2 levels correlated with increased ER-mediated expression of proteinase inhibitor-9 (PI-9), the only known intracellular inhibitor of the granzyme B pathway, while PI-9 inhibition blocked the protective effect of E2 against NK-mediated apoptosis (164). This evidence concerns the gene SERPINB9 and breast cancer.