TBB is a highly selective, ATP/GTP-competitive inhibitor of casein kinase-2 (CK2) (IC50 = 0.9 and 1.6 μM for rat liver and human recombinant CK2 respectively). TBB exhibits modest discrimination between CK2 subunits, with Ki values ranging from 80 nM to 210 nM. TBB acts in an ATP/GTP-competitive manner and displays one to two orders of magnitude selectivity over a panel of 33 protein kinases. The effect of TBB on PC-3 human prostate cancer cell viability depends on the time schedule of administration.