The Aurora kinases AurA, B and C are serine/threonine protein kinases that play essential roles in mitosis and cytokinesis. Among them, AurB is required for maintaining proper chromosome alignment, separation and segregation during mitosis, and regulating a number of critical processes involved in cytokinesis. AurB overexpression has been observed in a variety of cancer cell lines, and inhibition of AurB has been shown to induce tumour regression in mouse xenograft models. In the present study we report the enzymatic characterization of a potent and selective AurB/AurC inhibitor. GSK1070916 is a reversible and ATP-competitive inhibitor of the AurB-INCENP (inner centromere protein) enzyme. It selectively inhibits AurB-INCENP (K(i)*=0.38+/-0.29 nM) and AurC-INCENP (K(i)*=1.5+/-0.4 nM) over AurA-TPX2 (target protein for Xenopus kinesin-like protein 2) (K(i)=490+/-60 nM). Inhibition of AurB-INCENP and AurC-INCENP is time-dependent, with an enzyme-inhibitor dissociation half-life of >480 min and 270+/-28 min respectively. The extremely slow rate of dissociation from the AurB and AurC enzymes distinguishes GSK1070916 from two other Aurora inhibitors in the clinic, AZD1152 and VX-680 (also known as MK-0457).