A selective, irreversible inhibitor of BTK (IC50 = 0.5 nM in vitro in B cell lymphoma cell lines) that targets and covalently binds to BTK at cysteine-481; inhibits osteoclast function and reduces osteoclast-stimulated proliferation of multiple myeloma cells in animal models of rheumatoid arthritis and multiple sclerosis; demonstrates therapeutic significance in clinical trials for the treatment of B cell-related cancers and autoimmune diseases