An isolated nucleic acid sequence of a mitotic checkpoint gene, chfr, encodes a Chfr protein having a Forkhead-associated domain and a Ring Finger. This protein is required for regulation of the transition of cells from prophase to metaphase during mitosis. The chfr nucleic acid and Chfr polypeptide are useful in diagnosing tumorigenic cells and in screening for drugs which can inhibit the activity of Chfr in a cancer cell, thereby rendering the cell more sensitive to additional anti-tumor therapies.