Used in the treatment of several B-cell malignancies, including mantle cell lymphoma, chronic lymphocytic leukemia (CLL), and Waldenström's macroglobulinemia. It works by irreversibly inhibiting Bruton's tyrosine kinase (BTK), a key enzyme in B-cell receptor signaling, which plays a critical role in the survival and proliferation of malignant B cells. By blocking this pathway, the compound induces apoptosis and reduces tumor growth. Also investigated for use in other hematologic cancers and certain autoimmune conditions due to its modulation of immune cell activity.