CTLs and NK cells employ a myriad of strategies to destroy cancer cells,[6] including the secretion and delivery of serine proteases (e.g., granzymes).[7] Granzymes are stored in cytolytic granules,[8] with granzyme A (GzmA) and granzyme B (GzmB) being the most abundant subtypes.[7a] The expression levels of GzmA can be determined using antibodies,[9] but such constructs do not typically differentiate between the active and inactive forms of the enzyme. Here, GZMB is linked to cancer.