a b s t r a c tCdk1 plays an important role in undifferentiated ES cells, but the underlying mechanism remains unclear. This study explores how Cdk1 collaborates with Oct4 to inhibit differentiation in mouse ES cells. We show a direct interaction between Cdk1 and Oct4, whereas other Cdk members, including Cdk2 and Cdk4, fail to associate with Oct4. By immunocytochemistry we show that Cdk1 and Oct4 co-localize in ES cells. The biological function of the Cdk1-Oct4 complex was also addressed. We found that Cdk1 enhances the binding of Oct4 on the trophectoderm marker Cdx2 and promotes Cdx2 repression. This regulation is independent of Cyclins and of the kinase activity of Cdk1. Our study explains how Cdk1 and Oct4 interplay to inhibit ES cell differentiation into trophectoderm and thereby maintain stemness. Structured summary of protein interactions:Cdk1 physically interacts with Oct4 by anti tag coimmunoprecipitation (View interaction) Oct4 binds to SOX-2 by pull down (View interaction) Cdk1 physically interacts with Oct4 and cyclin-B1 by anti bait coimmunoprecipitation (View interaction) Oct4 binds to Cdk1 by pull down (View interaction) Cdk1 and Oct4 colocalize by fluorescence microscopy (View interaction) Oct4 physically interacts with Sox2 by anti bait coimmunoprecipitation (View interaction) Crown