The genetic material DNA is packaged and organized into chromosomes in cells. Abnormal numbers of chromosomes or changes in chromosomal structures or organizations will cause human diseases including cancers. Many mitotic checkpoint pathways are evolved in the cell to maintain chromosomal stability. Studies in the last decade have highlighted the importance of spindle assembly checkpoint in the maintenance of chromosomal stability. While spindle checkpoint clearly plays a critical role in this process, mutations or dysregulations of spindle checkpoint components are not frequently observed in cancers, suggesting that tumor-associated instability could result from other deficiencies. One of these deficiencies is likely to be Chfr downregulation, which has been repeatedly observed in many types of cancers. Checkpoint protein with FHA and RING domains (Chfr) is involved in early mitotic checkpoint. It comprises of a RING domain and functions as an E3 ubiquitin ligase. During the last funding period, we have established Chfr knockout mice and demonstrated directly that Chfr acts as tumor suppressor. In addition, we showed that the major function of Chfr is to regulate the stability of several key mitotic proteins including Aurora A and PLK1. Our current working hypothesis is that Chfr normally acts throughout G1, S and G2 phases, but is specifically downregulated in mitosis. The function of Chfr is to prevent premature accumulation of many key mitotic regulators before the entry of mitosis. In the absence of Chfr and probably due to untimely upregulation of these Chfr substrates, cells would undergo abnormal mitotic transitions, which result in low frequency of chromosomal instability and thus promote tumorigenesis. Therefore, the identification of Chfr substrates and the understanding of Chfr downregulation in mitosis will reveal how Chfr normally act to prevent tumor initiation in vivo. We recently identified a potential mitotic kinase TOPK as a Chfr substrate. In addition, we demonstrated that Chfr deficiency greatly promoted tumorigenesis in MLH1-deficient mice, suggesting that chromosomal instability caused by Chfr deficiency may synergize with microsatellite instability caused by mismatch repair deficiency to facilitate tumorigenesis. Based on these preliminary studies, we propose to: (1) Study the functional interaction between Chfr and TOPK in mitosis; (2) Explore whether balanced regulation of TOPK expression is critical for the maintenance of chromosomal stability and tumor suppression; (3) Determine whether chromosomal instability caused by Chfr downregulation cooperates with mismatch repair deficiency in tumor initiation.