Mob1p is an essential Saccharomyces cerevisiaeprotein, identified from a two-hybrid screen, that binds Mps1p, a protein kinase essential for spindle pole body duplication and mitotic checkpoint regulation. Mob1p contains no known structural motifs; however MOB1 is a member of a conserved gene family and shares sequence similarity with a nonessential yeast gene,MOB2. Mob1p is a phosphoprotein in vivo and a substrate for the Mps1p kinase in vitro. Conditional alleles ofMOB1 cause a late nuclear division arrest at restrictive temperature. MOB1 exhibits genetic interaction with three other yeast genes required for the completion of mitosis,LTE1, CDC5, and CDC15 (the latter two encode essential protein kinases). Most haploid mutantmob1 strains also display a complete increase in ploidy at permissive temperature. The mechanism for the increase in ploidy may occur through MPS1 function. One mob1strain, which maintains stable haploidy at both permissive and restrictive temperature, diploidizes at permissive temperature when combined with the mps1–1 mutation. Strains containingmob2Δ also display a complete increase in ploidy when combined with the mps1-1 mutation. Perhaps in addition to, or as part of, its essential function in late mitosis, MOB1 is required for a cell cycle reset function necessary for the initiation of the spindle pole body duplication.