A novel purified phosphoprotein designated mitosin is provided by this invention. Also provided is the amino acid sequence of mitosin, active fragments of mitosin, and a nucleic acid molecule encoding mitosin. Diagnostic and therapeutic methods of using the protein and nucleic acid molecule are also provided. The nucleic acid molecules are useful to recombinantly produce mitosin and for use as probes. The compositions and methods of this invention are based on the discovery that the intracellular presence of mitosin is necessary for the cell to enter the M phase of mitosis, and that the degradation of mitosin is necessary for the cell to advance to the next stage. Thus, an anti-mitsoin antibody, or a mutant or non-functional analog of mitosin, would inhibit the mitotic cell cycle by preventing cells from entering the M phase, and overexpression of mitosin, or a functional equivalent thereof, would inhibit the cycle by preventing cells from leaving the M phase. Such overexpression could be achieved either by addition of the protein or through gene therapy, i.e., delivery of a gene encoding the protein or a functional equivalent thereof.