BTB-1 blocks the motility of Kif18A in a reversible manner. BTB-1 inhibits Kif18A in an adenosine triphosphate (ATP)-competitive but microtubule-uncompetitive manner and slows down the progression of cells through mitosis. 100 μM BTB-1 does not significantly inhibit any of the other tested mitotic kinesins. BTB-1 competes with ATP for Kif18A binding only when the motor-protein is associated with its pseudosubstrate microtubules. HeLa cells treated with BTB-1 accumulate in mitosis in a dose-dependent manner. BTB-1 shows cell toxicity with an EC ₅₀ values of 35.8 μM. HeLa cells treated with 50 μM BTB-1 reveals severe defects in spindle morphology and chromosome alignment. Treatment with high concentrations of BTB-1 does not result in elongated spindles.