An orally bioavailable small-molecule inhibitor of Bruton's tyrosine kinase (BTK) with potential antineoplastic activity. BTK inhibitor PCI-32765 binds to and inhibits BTK activity, preventing B-cell activation and B-cell-mediated signaling and inhibiting the growth of malignant B cells that overexpress BTK. BTK, a member of the src-related BTK/Tec family of cytoplasmic tyrosine kinases, is required for B cell receptor (BCR) signaling, plays a key role in B-cell maturation, and is overexpressed in a number of B-cell malignancies.