In the process of investigating basic questions about the function of the enzyme phosphatidyl-ethanolamine N-methyltransferase (PEMT), we have made several unexpected findings. PEMT catalyzes the conversion of phosphatidylethanolamine to phosphatidylcholine in hepatocytes. A novel isoform, PEMT2, has been cloned, expressed and localized to a mitochondria-associated membrane in rat liver. Expression of PEMT2 in cultured rat hepatoma cells decreased the rate of cell division. Mechanistic studies suggest that the slower growth of transfected hepatoma cells is due to down regulation of CTP:phosphocholine cytidylyltransferase and the CDP-choline pathway for phosphatidylcholine biosynthesis. A role for PEMT2 in the regulation of hepatocyte cell division is also indicated by PEMT2 down-regulation in regenerating rat liver. Another unexpected finding was the discovery that PEMT2 is a liver specific tumor suppressor. Also surprising was the finding that expression of PEMT2 does not rescue mutant Chinese hamster ovary cells that have a temperature sensitive defect in the CDP-choline pathway even though the levels of phosphatidylcholine are returned to normal. Thus, curiosity-driven research has resulted in unexpected findings about PEMT and its function in liver.