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// Tatsuo Matsuda 1 , Taigo Kato 1 , Kazuma Kiyotani 1 , Yunus Emre Tarhan 1 , Vassiliki Saloura 1 , Suyoun Chung 2 , Koji Ueda 3 , Yusuke Nakamura 1,4 and Jae-Hyun Park 1 1 Department of Medicine, The University of Chicago, Chicago, IL, USA 2 OncoTherapy Science Inc., Kawasaki, Japan 3 Project for Realization of Personalized Cancer Medicine, Cancer Precision Medicine Center, Japanese Foundation for Cancer Research, Tokyo, Japan 4 Department of Surgery, The University of Chicago, Chicago, IL, USA Correspondence to: Yusuke Nakamura, email: // Keywords : maternal embryonic leucine zipper kinase, molecular target, p21, p53, FoxO family Received : April 12, 2017 Accepted : June 01, 2017 Published : June 15, 2017 Abstract MELK play critical roles in human carcinogenesis through activation of cell proliferation, inhibition of apoptosis and maintenance of stemness. Therefore, MELK is a promising therapeutic target for a wide range of cancers. Although p21 is a well-known p53-downstream gene, we found that treatment with a potent MELK inhibitor, OTS167, could induce p21 protein expression in cancer cell lines harboring loss-of-function TP53 mutations. We also confirmed that MELK knockdown by siRNA induced the p21 expression in p53-deficient cancer cell lines and caused the cell cycle arrest at G1 phase. Further analysis indicated that FOXO1 and FOXO3, two known transcriptional regulators of p21, were phosphorylated by MELK and thus be involved in the induction of p21 after MELK inhibition. Collectively, our herein findings suggest that MELK inhibition may be effective for human cancers even if TP53 is mutated.