CTP:phosphocholine cytidylyltransferase (CT) is the rate-limiting enzyme in the CDP-choline pathway for the de novo synthesis of phosphatidylcholine (PC) in the cytoplasm. However over 95 percent of CT is not required for PC synthesis and is present in the nucleus for unknown reasons. Recent studies show that CT also play an important role in the regulation of DNA ploidy. Deficiency of CT can activate the overexpressed B cell lymphoma gene 2 protein (Bcl-2p) to induce the formation of stable and viable tetraploid cells from diplod cells. Expression of CT can suppress the formation of tetraploid cells induced by Bcl-2p. The long-term goal of this proposal is to understand how CT is involved in the regulation of genome duplication. We will test the hypothesis that CT may have a second role, different from catalyzing CDP-choline formation, to regulate the genome duplication. The first specific aim will determine whether this abnormal genome duplication occurs by which one of the following mechanisms: 1) two rounds of DNA synthesis in one cell cycle; 2) failure of mitosis between two S phases or 3) cell fusion between two parental cells. The second specific aim is to determine whether the noncytalytic properties are important for CT to regulate the Bcl-2p-induced genome duplication by studying the CT deletion mutants lacking these properties. The third specific aim will determine whether the catalytic activity for the synthesis of CDP- choline is important for CT in the regulation of genome duplication by studying the inactive CT mutants with point mutation at the catalytic domain. Being active in both PC synthesis and the regulation of genome duplication suggests that CT may coordinate PC synthesis with cell division. The proposed studies will provide insights into 1) the mechanism of polyploidy in cancer cells; 2) the coordination between ploidy control and PC synthesis; 3) a novel role of Bcl-2 in promoting polyploidy. A better understanding of the mechanism that underlines the genome instability in cancer cells will be helpful for designing more efficient strategies of treating cancer.