The present invention provides a threonine/tyrosine kinase, Myt1, that phosphorylates the cyclin-dependent kinase, Cdc2, to provide regulation of the cell cycle at the G2/M interphase. Also included are polynucleotides encoding Myt1 polypeptide and antibodies that bind to Myt1. Methods of modulating Myt1 for preventing premature entry of the cell into mitosis or to accelerate entry into mitosis are also provided, as are methods for identifying agents that modulate Myt1.