The co-chaperone murine stress-inducible protein 1 (mSTI1), a Hsp70/Hsp90 organizing protein (Hop) homolog, functions as a physical link between Hsp70 and Hsp90 by mediating the formation of the mSTI1/ Hsp70/Hsp90 chaperone heterocomplex. We show here that mSTI1 is an in vitro substrate of cell cycle kinases. Casein kinase II (CKII) phosphorylates mSTI1 at S189, and cdc2 kinase (p34cdc2) at T198, substantiating a predicted CKII-p34cdc2-NLS (CcN) motif. The possible implications of this phosphorylation as a cell cycle checkpoint are discussed.