MBM-55S is a potent NIMA-related kinase 2 ( Nek2 ) inhibitor with an IC 50 of 1 nM. MBM-55S shows a 20-fold or greater selectivity in most kinases with the exception of RSK1 ( IC 50 =5.4 nM) and DYRK1a ( IC 50 =6.5 nM). MBM-55S effectively inhibits the proliferation of cancer cells by inducing cell cycle arrest and apoptosis. MBM-55S shows antitumor activities, and no obvious toxicity to mice.